Opening an existing Asp.Net 1.1 solution on my development machine resulted in this error message. Restarting IIS didn’t resolve it, nor did repairing the .NET installation. In combination, though, as suggested by NewGuy in this old post did the trick. I had to stop IIS, delete the asp.net user, repair the framework installation, then restart IIS.
First time for this particular problem. Thanks to experienced people contributing to public fora.
Eta: deleting the asp.net user.