Perhaps the logon form has become corrupt, or the table it’s bound to (if any).
Start by making a backup copy of the database.
Try this first:
– Open the database with Shift held down.
– Select Tools | Database Utilities | Compact and Repair Database.
If that doesn’t help:
– Create a blank new database.
– Import all objects from the problem database.
– If one ore more objects can’t be imported, they are the culprit(s). See if you can import them from an older non-corrupt backup copy.
If that doesn’t help either:
– If you can find the database object that causes the problem, try to delete it, then to re-create it from scratch.
– Or go back to an older non-corrupt backup copy.