Thank you for the detailed information .... you seem to think the company files need verification/repair, which I've done .... but I'm still concerned about the .NET Framework errors that show up in the event log. The errors associated with File Doctor occur prior to a company file even being opened.  I tried removing .NET and letting Quickbooks install .NET when it was launched but that didn't help.  Is there a specific version of .NET or QB 2021 that we know is compatible, and how can I verify that?

But the DB Manager is installed.  Then File Doctor throws this message:

************ Exception Text **************
System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send. ---> System.IO.IOException: Received an unexpected EOF or 0 bytes from the transport stream.