I want to store encrypted credentials and use them to open a powershell instance and run a script that makes a change to a field in AD

Question

Below is my code, I've used the same process for connecting to sftp securely. I'm getting the error at the bottom of my post. Not sure if I'm missing a step in the creation of the key and password. Thanks.

#Set the credentials
$Password = Get-Content "c:\password.txt" | 
ConvertTo-SecureString -Key (Get-Content "c:\aes.key")
$Credential = New-Object System.Management.Automation.PSCredential ('serviceaccount', $Password)

# Start a new instance of Windows PowerShell using the credentials 
# stored in $Credential and run the script in $scriptblock
$powershellPath = "$env:windir\system32\windowspowershell\v1.0\powershell.exe"
$process = Start-Process $powershellPath -Credential $Credential -NoNewWindow `
-ArgumentList ("-ExecutionPolicy Bypass -noninteractive -noprofile " + $scriptBlock) -PassThru

# Script to execute in the new PowerShell instance
$scriptBlock = {
    Import-Module ActiveDirectory
    Get-ADUser ecarlsson | Set-ADUser -Manager bbob

    Read-Host    
}

I tried the code above and go the password error below.

Start-Process : This command cannot be run due to the error: The user name or password is incorrect.
At\filepath \\fV3.ps1:7 char:12
+ $process = Start-Process $powershellPath -Credential $Credential -NoN ...
+            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [Start-Process], InvalidOperationException
    + FullyQualifiedErrorId : InvalidOperationException,Microsoft.PowerShell.Commands.StartProcessCommand