Cause 1: App pool user isn’t setup in Trusted web service account maintenance screen
Resolution: Add the application pool user to the Trusted web service account maintenance screen
Cause 2: web apps user is not setup in the user maintenance screen
Resolution: Setup use in the User Maintenance screen
Cause 3: old web services dll file exists in the C:\windows\assembly
Resolution: Copy the webservices.dll file from the new installation to the C:\windows\assembly
Cause 4: Issue with userrec.defaultuser field
Resolution: Delete the user and add them back to the system in User Maintenance