"Oh darn... it's one of those again". That was there first thing I thought recently when all-of-a-sudden I got an error in VS2008 after trying to open up a generated DBML file in the Object Relational Designer:

"The operation could not be completed."

Now that's helpful, isn't it? After a bit of digging in the output window I also found this one:

"The custom tool 'MSLinqToSQLGenerator' failed.  Could not retrieve the current project."

The part about the custom tool lead me to think it would probably be an issue with an AddIn. But unfortunately, the AddIn manager in VS2008 doesn't have an entry for this tool. But still this was close to the solution. Somehow, somewhere (in a galaxy far, far away) Visual Studio encountered an error in the tool and must have marked it not be started again. Ever.

Anyway: In the registry, I deleted all subkeys below HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\9.0\Packages and it worked like a blast again.

Thanks to this post on the MSDN forums.