If the error occurs, restart PaperOffice first and check if the error has not been fixed. If the error still persists:
You should try to isolate the problem by disconnecting the database connection. In your login window, click on Options and select "Disconnect the database connection".
Now create a new empty database and check, if the error should occur even with a new empty database by adding a test document to this database. If the bug was fixed by the new database, connect again to your existing database, which caused the error and create a backup. Restart PaperOffice and disconnect the failed database again.
Connect now to the backed up database and see if the problem is still there.