DNN Upgrade Module From .Net 3.5 to .Net 4.6.1

Question

I have multiple DNN modules built on .Net 3.5 and DNN 5.2, I upgraded them to DNN 8.1 & .Net 4.6.1, so in the upgraded CMS, some modules are working properly, and others not working (not working without showing any error message or any exception).

To upgrade my modules, I followed these steps:

1. Changed the Target framework of each module project to be 4.6.1.
2. Fix build errors which raised after changing the Target Framework.
3. Build the module project successfully.
4. Copy and replace the file bin/release/module_name.dll to Portal/bin folder

Note: for those modules which not working, I can't find any error message describing that I have an issue in the code or something can't be loaded !

So probably made a mistake or missing some other steps?

Update:

After checking the Admin -> Event Viewer, I've found that after dropping the module object inside the page, a General Exception record is inserted to the errors says:

StackTrace -> at DotNetNuke.Services.Search.ModuleIndexer.ThrowLogError(ModuleInfo module, Exception ex)

InnerMessage -> Value cannot be null. Parameter name: type

InnerStackTrace ->
at System.Activator.CreateInstance(Type type, Boolean nonPublic)

at System.Activator.CreateInstance(Type type)

at DotNetNuke.Services.Search.ModuleIndexer.GetModuleList(Int32 portalId)