Cause: Corruption of the Library database stored in the windows user profile. Changes in the windows security environment, updates, or installation of a new software may trigger this issue. The causes are not entirely clear.
Solution: The solution is to force the Librarian to create a brand new database library or delete the old database (*.db) files and restore them from a backup.
The database (*.db) files are located in the Roaming user profile %appdata%\Librarian\<YYYY>\_libdb\ folder. One can simply delete the folder or rename it so that the Librarian can re-create the folder when Revit is restarted again.
