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VS2017 crashes with 'FileNotFoundEx: System.Runtime.CompilerServices.Unsafe, V=4.0.4.1' upon loading any project

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Sorry for a lengthy one, but I'm in dire straits - just trying to provide all details upfront.

This Fri (2021-Nov-12) after a restart of Visual Studio 2017 it began crashing without notice while opening existing solutions. This worked perfectly fine at least a week ago (after last Win10 Update KB5006670 on 2021-Nov-05 - followed by a reboot). Trying to load old solutions (which haven't been touched for 2+ years) results in exactly the same behavior:
you get a glimpse of "Loading Project .." windows (not sure if it goes through all projects in a solution), then suddenly the main VS window disappears and .. that's it.

VStudio's configuration has not been touched at least for a year. No explicit updates/patches or NuGet packages either. By itself VS starts and shows the main window with usual Start page. But I cannot load any solution or project.

The very first related Event Log entry:

**2021-Nov-12 14:13:41**
Application: devenv.exe
Framework Version: v4.0.30319
Description: The application requested process termination through System.Environment.FailFast(string message).
Message: System.IO.FileNotFoundException: Could not load file or assembly 'System.Runtime.CompilerServices.Unsafe, Version=4.0.4.1, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.
File name: 'System.Runtime.CompilerServices.Unsafe, Version=4.0.4.1, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
   at System.Threading.Tasks.ValueTask`1.AsTask()
   at Microsoft.CodeAnalysis.Document.GetSyntaxTreeAsync(CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.Host.BackgroundParser.<>c__DisplayClass20_0.<ParseDocumentAsync>b__0()
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()

WRN: Assembly binding logging is turned OFF.
To enable assembly bind failure logging, set the registry value [HKLM\Software\Microsoft\Fusion!EnableLog] (DWORD) to 1.
Note: There is some performance penalty associated with assembly bind failure logging.
To turn this feature off, remove the registry value [HKLM\Software\Microsoft\Fusion!EnableLog].

Stack:
   at System.Environment.FailFast(System.String, System.Exception)
   at Microsoft.CodeAnalysis.FailFast.OnFatalException(System.Exception)
   at Microsoft.CodeAnalysis.ErrorReporting.FatalError.Report(System.Exception, System.Action`1<System.Exception>)
   at Microsoft.CodeAnalysis.ErrorReporting.FatalError.Report(System.Exception)
   at Roslyn.Utilities.TaskExtensions.ReportFatalErrorWorker(System.Threading.Tasks.Task, System.Object)
   at System.Threading.Tasks.ContinuationTaskFromTask.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
   at System.Threading.Tasks.Task.ExecutionContextCallback(System.Object)
   at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef)
   at System.Threading.Tasks.Task.ExecuteEntry(Boolean)
   at System.Threading.Tasks.ThreadPoolTaskScheduler.TryExecuteTaskInline(System.Threading.Tasks.Task, Boolean)
   at System.Threading.Tasks.TaskScheduler.TryRunInline(System.Threading.Tasks.Task, Boolean)
   at System.Threading.Tasks.TaskContinuation.InlineIfPossibleOrElseQueue(System.Threading.Tasks.Task, Boolean)
   at System.Threading.Tasks.StandardTaskContinuation.Run(System.Threading.Tasks.Task, Boolean)
   at System.Threading.Tasks.Task.FinishContinuations()
   at System.Threading.Tasks.Task.FinishStageThree()
   at System.Threading.Tasks.Task.FinishStageTwo()
   at System.Threading.Tasks.Task.Finish(Boolean)
   at System.Threading.Tasks.Task`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].TrySetException(System.Object)
   at System.Threading.Tasks.UnwrapPromise`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].TrySetFromTask(System.Threading.Tasks.Task, Boolean)
   at System.Threading.Tasks.UnwrapPromise`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].ProcessCompletedOuterTask(System.Threading.Tasks.Task)
   at System.Threading.Tasks.UnwrapPromise`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].InvokeCore(System.Threading.Tasks.Task)
   at System.Threading.Tasks.UnwrapPromise`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].Invoke(System.Threading.Tasks.Task)
   at System.Threading.Tasks.Task.FinishContinuations()
   at System.Threading.Tasks.Task.FinishStageThree()
   at System.Threading.Tasks.Task.FinishStageTwo()
   at System.Threading.Tasks.Task.Finish(Boolean)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef)
   at System.Threading.Tasks.Task.ExecuteEntry(Boolean)
   at System.Threading.Tasks.Task.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

was followed by these 3:

1)

**2021-Nov-12 14:14:16**
Application: devenv.exe
Framework Version: v4.0.30319
Description: The application requested process termination through System.Environment.FailFast(string message).
Message: System.IO.FileNotFoundException: Could not load file or assembly 'System.Runtime.CompilerServices.Unsafe, Version=4.0.4.1, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.
File name: 'System.Runtime.CompilerServices.Unsafe, Version=4.0.4.1, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
   at System.Threading.Tasks.ValueTask`1.AsTask()
   at Microsoft.CodeAnalysis.Document.GetSyntaxTreeAsync(CancellationToken cancellationToken)
   at Microsoft.VisualStudio.LanguageServices.Implementation.Progression.SymbolContainment.<GetContainedSyntaxNodesAsync>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.VisualStudio.LanguageServices.Implementation.Progression.ContainsChildrenGraphQuery.<GetGraphAsync>d__0.MoveNext()

WRN: Assembly binding logging is turned OFF.
To enable assembly bind failure logging, set the registry value [HKLM\Software\Microsoft\Fusion!EnableLog] (DWORD) to 1.
Note: There is some performance penalty associated with assembly bind failure logging.
To turn this feature off, remove the registry value [HKLM\Software\Microsoft\Fusion!EnableLog].

Stack:
   at System.Environment.FailFast(System.String, System.Exception)
   at Microsoft.CodeAnalysis.FailFast.OnFatalException(System.Exception)
   at Microsoft.CodeAnalysis.ErrorReporting.FatalError.Report(System.Exception, System.Action`1<System.Exception>)
   at Microsoft.CodeAnalysis.ErrorReporting.FatalError.ReportUnlessCanceled(System.Exception)
   at Roslyn.Utilities.TaskExtensions+<>c__DisplayClass7_0`1[[System.Boolean, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].<SafeContinueWith>g__outerFunction|0(System.Threading.Tasks.Task)
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean)
   at System.Threading.Tasks.Task`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].GetResultCore(Boolean)
   at System.Threading.Tasks.Task`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].get_Result()
   at Microsoft.VisualStudio.LanguageServices.Implementation.Progression.GraphQueryManager+<>c__DisplayClass14_0.<PopulateContextGraphAsync>b__1(System.Threading.Tasks.Task`1<Microsoft.VisualStudio.LanguageServices.Implementation.Progression.GraphBuilder[]>)
   at Roslyn.Utilities.TaskExtensions+<>c__DisplayClass6_0`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].<SafeContinueWith>b__0(System.Threading.Tasks.Task)
   at Roslyn.Utilities.TaskExtensions+<>c__DisplayClass3_0.<SafeContinueWith>g__continuationFunction|0(System.Threading.Tasks.Task)
   at Roslyn.Utilities.TaskExtensions+<>c__DisplayClass7_0`1[[System.Boolean, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].<SafeContinueWith>g__outerFunction|0(System.Threading.Tasks.Task)
   at System.Threading.Tasks.ContinuationResultTaskFromTask`1[[System.Boolean, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
   at System.Threading.Tasks.Task.ExecutionContextCallback(System.Object)
   at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef)
   at System.Threading.Tasks.Task.ExecuteEntry(Boolean)
   at System.Threading.Tasks.Task.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

**2021-Nov-12 14:14:19**
Faulting application name: devenv.exe, version: 15.8.28010.2050, time stamp: 0x5bda1fc3
Faulting module name: unknown, version: 0.0.0.0, time stamp: 0x00000000
Exception code: 0x80131623
Fault offset: 0x19a1c93a
Faulting process id: 0x3d44
Faulting application start time: 0x01d7d801d444a508
Faulting application path: C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\Common7\IDE\devenv.exe
Faulting module path: unknown
Report Id: e230b637-fd72-47f8-a0d5-d2c4ccb10943

**2021-Nov-12 14:14:35**
Fault bucket 1659198495851531123, type 5
Event Name: CLR20r3
Response: Not available
Cab Id: 0

Problem signature:
P1: devenv.exe
P2: 15.8.28010.2050
P3: 5bda1fc3
P4: Microsoft.CodeAnalysis.Workspaces
P5: 2.9.0.63208
P6: f144aff0
P7: 500
P8: 41
P9: System.IO.FileNotFoundException
P10:

This pattern of 3 entries with same suspects repeated several times while I was trying to load solutions.

In the last triple it had ServiceHub.RoslynCodeAnalysisService32.exe as the faulting app:

**2021-Nov-12 14:28:05**
Application: ServiceHub.RoslynCodeAnalysisService32.exe
Framework Version: v4.0.30319
Description: The application requested process termination through System.Environment.FailFast(string message).
Message: System.OperationCanceledException: The operation was canceled.
   at System.Threading.CancellationToken.ThrowOperationCanceledException()
   at Microsoft.CodeAnalysis.Remote.Extensions.<InvokeAsync>d__3`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.Remote.ServiceHubServiceBase.<RunServiceAsync>d__25`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.Remote.SnapshotService.JsonRpcAssetSource.<RequestAssetsAsync>d__2.MoveNext()
Stack:
   at System.Environment.FailFast(System.String, System.Exception)
   at Microsoft.CodeAnalysis.FailFast.OnFatalException(System.Exception)
   at Microsoft.CodeAnalysis.Remote.SnapshotService+JsonRpcAssetSource.ReportUnlessCanceled(System.Exception, System.Threading.CancellationToken)
   at Microsoft.CodeAnalysis.Remote.SnapshotService+JsonRpcAssetSource+<RequestAssetsAsync>d__2.MoveNext()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(System.Threading.Tasks.Task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(System.Threading.Tasks.Task)
   at Microsoft.CodeAnalysis.Remote.SnapshotService+JsonRpcAssetSource+<RequestAssetsAsync>d__2.MoveNext()
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore+MoveNextRunner.InvokeMoveNext(System.Object)
   at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore+MoveNextRunner.Run()
   at System.Threading.Tasks.AwaitTaskContinuation.RunOrScheduleAction(System.Action, Boolean, System.Threading.Tasks.Task ByRef)
   at System.Threading.Tasks.Task.FinishContinuations()
   at System.Threading.Tasks.Task.FinishStageThree()
   at System.Threading.Tasks.Task.CancellationCleanupLogic()
   at System.Threading.Tasks.Task`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].TrySetCanceled(System.Threading.CancellationToken, System.Object)
   at System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].SetException(System.Exception)
   at Microsoft.CodeAnalysis.Remote.ServiceHubServiceBase+<RunServiceAsync>d__25`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].MoveNext()
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore+MoveNextRunner.InvokeMoveNext(System.Object)
   at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore+MoveNextRunner.Run()
   at System.Threading.Tasks.AwaitTaskContinuation.RunOrScheduleAction(System.Action, Boolean, System.Threading.Tasks.Task ByRef)
   at System.Threading.Tasks.Task.FinishContinuations()
   at System.Threading.Tasks.Task.FinishStageThree()
   at System.Threading.Tasks.Task.CancellationCleanupLogic()
   at System.Threading.Tasks.Task`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].TrySetCanceled(System.Threading.CancellationToken, System.Object)
   at System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].SetException(System.Exception)
   at Microsoft.CodeAnalysis.Remote.Extensions+<InvokeAsync>d__3`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].MoveNext()
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore+MoveNextRunner.InvokeMoveNext(System.Object)
   at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore+MoveNextRunner.Run()
   at System.Threading.Tasks.AwaitTaskContinuation.RunOrScheduleAction(System.Action, Boolean, System.Threading.Tasks.Task ByRef)
   at System.Threading.Tasks.Task.FinishContinuations()
   at System.Threading.Tasks.Task.FinishStageThree()
   at System.Threading.Tasks.Task.CancellationCleanupLogic()
   at System.Threading.Tasks.Task`1[[System.Threading.Tasks.VoidTaskResult, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].TrySetCanceled(System.Threading.CancellationToken, System.Object)
   at System.Threading.Tasks.TaskFactory`1[[System.Threading.Tasks.VoidTaskResult, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].FromAsyncCoreLogic(System.IAsyncResult, System.Func`2<System.IAsyncResult,System.Threading.Tasks.VoidTaskResult>, System.Action`1<System.IAsyncResult>, System.Threading.Tasks.Task`1<System.Threading.Tasks.VoidTaskResult>, Boolean)
   at System.Threading.Tasks.TaskFactory`1+<>c__DisplayClass35_0[[System.Threading.Tasks.VoidTaskResult, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].<FromAsyncImpl>b__0(System.IAsyncResult)
   at System.IO.Pipes.NamedPipeServerStream.AsyncWaitForConnectionCallback(UInt32, UInt32, System.Threading.NativeOverlapped*)
   at System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32, UInt32, System.Threading.NativeOverlapped*)

**2021-Nov-12 14:28:06**
Faulting application name: ServiceHub.RoslynCodeAnalysisService32.exe, version: 1.3.77.18573, time stamp: 0xdc9a59bf
Faulting module name: unknown, version: 0.0.0.0, time stamp: 0x00000000
Exception code: 0x80131623
Fault offset: 0x0aa234ca
Faulting process id: 0x2d4c
Faulting application start time: 0x01d7d803c7d13c18
Faulting application path: C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\Common7\ServiceHub\Hosts\ServiceHub.Host.CLR.x86\ServiceHub.RoslynCodeAnalysisService32.exe
Faulting module path: unknown

**2021-Nov-12 14:28:11**
Fault bucket 1674005537433911736, type 5
Event Name: CLR20r3
Response: Not available
Cab Id: 0

Problem signature:
P1: O5DFLQX35YXZVV3T1Q5XYRCHNTQNLFQ2
P2: 1.3.77.18573
P3: dc9a59bf
P4: mscorlib
P5: 4.8.4420.0
P6: 6109cb33
P7: 3e6d
P8: 15
P9: System.OperationCanceled
P10:

The last one (today's first):

**2021-Nov-15 09:45:25**
Application: devenv.exe
Framework Version: v4.0.30319
Description: The application requested process termination through System.Environment.FailFast(string message).
Message: System.IO.FileNotFoundException: Could not load file or assembly 'System.Runtime.CompilerServices.Unsafe, Version=4.0.4.1, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.
File name: 'System.Runtime.CompilerServices.Unsafe, Version=4.0.4.1, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
   at System.Threading.Tasks.ValueTask`1.AsTask()
   at Microsoft.CodeAnalysis.Document.GetSyntaxTreeAsync(CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.Host.BackgroundParser.<>c__DisplayClass20_0.<ParseDocumentAsync>b__0()
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()

WRN: Assembly binding logging is turned OFF.
To enable assembly bind failure logging, set the registry value [HKLM\Software\Microsoft\Fusion!EnableLog] (DWORD) to 1.
Note: There is some performance penalty associated with assembly bind failure logging.
To turn this feature off, remove the registry value [HKLM\Software\Microsoft\Fusion!EnableLog].

Stack:
   at System.Environment.FailFast(System.String, System.Exception)
   at Microsoft.CodeAnalysis.FailFast.OnFatalException(System.Exception)
   at Microsoft.CodeAnalysis.ErrorReporting.FatalError.Report(System.Exception, System.Action`1<System.Exception>)
   at Microsoft.CodeAnalysis.ErrorReporting.FatalError.Report(System.Exception)
   at Roslyn.Utilities.TaskExtensions.ReportFatalErrorWorker(System.Threading.Tasks.Task, System.Object)
   at System.Threading.Tasks.ContinuationTaskFromTask.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
   at System.Threading.Tasks.Task.ExecutionContextCallback(System.Object)
   at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef)
   at System.Threading.Tasks.Task.ExecuteEntry(Boolean)
   at System.Threading.Tasks.ThreadPoolTaskScheduler.TryExecuteTaskInline(System.Threading.Tasks.Task, Boolean)
   at System.Threading.Tasks.TaskScheduler.TryRunInline(System.Threading.Tasks.Task, Boolean)
   at System.Threading.Tasks.TaskContinuation.InlineIfPossibleOrElseQueue(System.Threading.Tasks.Task, Boolean)
   at System.Threading.Tasks.StandardTaskContinuation.Run(System.Threading.Tasks.Task, Boolean)
   at System.Threading.Tasks.Task.FinishContinuations()
   at System.Threading.Tasks.Task.FinishStageThree()
   at System.Threading.Tasks.Task.FinishStageTwo()
   at System.Threading.Tasks.Task.Finish(Boolean)
   at System.Threading.Tasks.Task`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].TrySetException(System.Object)
   at System.Threading.Tasks.UnwrapPromise`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].TrySetFromTask(System.Threading.Tasks.Task, Boolean)
   at System.Threading.Tasks.UnwrapPromise`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].ProcessCompletedOuterTask(System.Threading.Tasks.Task)
   at System.Threading.Tasks.UnwrapPromise`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].InvokeCore(System.Threading.Tasks.Task)
   at System.Threading.Tasks.UnwrapPromise`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].Invoke(System.Threading.Tasks.Task)
   at System.Threading.Tasks.Task.FinishContinuations()
   at System.Threading.Tasks.Task.FinishStageThree()
   at System.Threading.Tasks.Task.FinishStageTwo()
   at System.Threading.Tasks.Task.Finish(Boolean)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef)
   at System.Threading.Tasks.Task.ExecuteEntry(Boolean)
   at System.Threading.Tasks.Task.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

Roslyn.Utilities.TaskExtensions.ReportFatalErrorWorker() seems to be a common fault catcher, so my guess is the issue has smth to do with background code analysis, launched upon loading a solution. Who broke it and how?

Searching for the error message System.IO.FileNotFoundException: Could not load file or assembly 'System.Runtime.CompilerServices.Unsafe, Version=4.0.4.1, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified. gave me the following links: 1, 2, 3, 4, 5.
But none of those describe my scenario. I do not have any [known] dependency on that library in any of my projects. Until Fri I wasn't even aware of it!

Prior to trying a possible remedy, suggested in 3's accepted answer ("the error went away after I ran a NuGet update for that DLL"), I want to understand, what the hell happened (and how!)? I do not have an explicit entry for such NuGet package in my list. I did not install or change anything recently.
This fiasco is precisely the reason why I try to keep rarely-changing [=> known & stable] development environment.

Searching for System.Runtime.CompilerServices.Unsafe.dll on the HDD produces the following:

C:\>dir System.Runtime.CompilerServices.Unsafe.* /s /n

C:\Program Files\dotnet\sdk\2.1.403\DotnetTools\dotnet-sql-cache\2.1.1\tools\netcoreapp2.1\any
    2018-Sep-20  14:48            14,712

C:\Program Files\dotnet\sdk\2.1.403\DotnetTools\dotnet-user-secrets\2.1.1\tools\netcoreapp2.1\any
    2018-Sep-20  14:48            14,928

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.4.0
    2018-Nov-08  16:14            62,144 system.runtime.compilerservices.unsafe.4.4.0.nupkg

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.4.0\lib\netstandard1.0
    2018-Nov-08  16:14            21,792

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.4.0\lib\netstandard2.0
    2018-Nov-08  16:14            21,944

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.4.0\ref\netstandard1.0
    2018-Nov-08  16:14            22,968

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.4.0\ref\netstandard2.0
    2018-Nov-08  16:14            23,480

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.5.0
    2018-Nov-08  16:10            87,453 system.runtime.compilerservices.unsafe.4.5.0.nupkg

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.5.0\lib\netcoreapp2.0
    2018-Nov-08  16:10            21,648

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.5.0\lib\netstandard1.0
    2018-Nov-08  16:10            22,160

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.5.0\lib\netstandard2.0
    2018-Nov-08  16:10            22,160

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.5.0\ref\netstandard1.0
    2018-Nov-08  16:10            23,176

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.5.0\ref\netstandard2.0
    2018-Nov-08  16:10            23,696

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.5.1
    2018-Nov-08  16:10            87,219 system.runtime.compilerservices.unsafe.4.5.1.nupkg

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.5.1\lib\netcoreapp2.0
    2018-Nov-08  16:11            21,640

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.5.1\lib\netstandard1.0
    2018-Nov-08  16:11            22,160

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.5.1\lib\netstandard2.0
    2018-Nov-08  16:11            22,184

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.5.1\ref\netstandard1.0
    2018-Nov-08  16:11            23,720

C:\Program Files\dotnet\sdk\NuGetFallbackFolder\system.runtime.compilerservices.unsafe\4.5.1\ref\netstandard2.0
    2018-Nov-08  16:11            24,208

C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App\2.1.5
    2018-Sep-20  15:08            19,536        4.6.26919.2

C:\Program Files\dotnet\store\x64\netcoreapp2.0\system.runtime.compilerservices.unsafe\4.4.0\lib\netstandard2.0
    2017-Jul-25  00:57            17,416

C:\Program Files\WindowsApps\Microsoft.YourPhone_1.21092.149.0_x64__8wekyb3d8bbwe
    2021-Jul-01  06:45            16,768        4.6.28619.1

C:\Program Files\WindowsApps\Microsoft.YourPhone_1.21092.149.0_x64__8wekyb3d8bbwe\YourPhoneAppProxy
    2021-Oct-19  05:48            18,824        4.700.21.47101

C:\Program Files\WindowsApps\Microsoft.YourPhone_1.21092.149.0_x64__8wekyb3d8bbwe\YourPhoneServer
    2020-Jun-13  17:25            17,000        4.700.20.12001

C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\Common7\IDE\Extensions\001ss942.rgs
    2021-May-20  04:28            16,768        5.0.20.51904            [RedGate.SQLSearch ?]

C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\Common7\IDE\Extensions\ph2houvf.kuk
    2019-Sep-07  04:28            21,792        ?? no version ??        [MS VS CloudExplorer]

VStudio's Help About shows the following version numbers:

Microsoft Visual Studio Professional 2017 
Version 15.8.9
VisualStudio.15.Release/15.8.9+28010.2050
Microsoft .NET Framework
Version 4.8.04084

ASP.NET and Web Tools 2017   15.8.05085.0
ASP.NET Core Razor Language Services   15.8.31590
ASP.NET Web Frameworks and Tools 2017   5.2.60618.0
C# Tools   2.9.0-beta8-63208-01
NuGet Package Manager   4.6.0
Redgate SQL Search   3.5.5.2703
SQL Server Data Tools   15.1.61808.07020
Xamarin   4.11.0.779 (d15-8@ff915e800)
Xamarin Designer   4.15.12 (d7ff6f39c)
Xamarin Templates   1.1.118 (4217ee9)
Xamarin.Android SDK   9.0.0.19 (HEAD/a8a3b0ec7)
c# visual-studio-2017 roslyn roslyn-code-analysis compiler-services
Original Q&A
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There are 1 best solutions below

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On BEST ANSWER

Sorry it took so long. Was under a gun to finish a project..

The root cause of the problem turned out to be ICSharpCode.CodeConverter v.8.4.1.0!
Wow, of all the pieces installed (which aren't that many)..

On a hunch (since the problem was local to Visual Studio) I started looking at Tools and Extensions, and noticed on this component the Date Installed being past the most recent Windows Update! The Automatically update this extension checkbox was checked (by default?).
So it must have silently updated upon VS restart?!

Granted, updates are useful and sometimes necessary. But they also may introduce problems. Performing updates automatically is one thing. But not informing the user about it is bad!

Here's an excerpt from the C:\TEMP\VSIXInstaller_f0335270-1a19-4b71-b74b-e50511bcd107.log:

Initializing Uninstall...
Extension Details...
    Identifier         : 7e2a69d6-193b-4cdf-878d-3370d5931942
    Name               : Code Converter (VB - C#)
    Author             : IC#Code
    Version            : 8.4.1.0
    Description        : Convert VB.NET to C# and vice versa with this roslyn based converter
    Locale             : en-US
    MoreInfoURL        : https://github.com/icsharpcode/CodeConverter
    InstalledByMSI     : False
    SupportedFrameworkVersionRange : [4.6,)

    Supported Products : 
        Microsoft.VisualStudio.Community
            Version : [15.0,17.0)
        Microsoft.VisualStudio.Community
            Version : [17.0,18.0)

    References         : 
    Prerequisites      : 
        -------------------------------------------------------
        Identifier   : Microsoft.VisualStudio.Component.CoreEditor
        Name         : Visual Studio core editor
        Version      : [15.0.26004.1,)

As soon as I removed this, VStudio was able to open/load projects; no restart was needed! Before it would crash even initializing/opening a new empty ones!

While reading the documentation on this converter I also found a very similar bug.

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