Hi - I encountered this the other day, and no information could be found at Microsoft.
My situation:
DB on network drive, used constantly by two users. A few days ago, they came in in the morning and tried to open the DB and received the "Unexpected Error: 35012". The switchboard would not open but it would open the database window.
The problem seemed to be connected to the switchboard, because if it was disabled, everything worked fine. However, upon further poking around, I traced the problem to a single form and a single table. I identified these by creating a blank database and importing all objects from the corrupt database over. The import gave a locking error on the form and table that were causing the problem.
I recreated this table (had to copy the format, and then cut and paste the data) and the form, and the issue was fixed.
I also changed the record locking from 'no locks' to 'edited record' locks. I have a funny feeling that this may have been responsible (Access gets pretty screwy with 'no locks' on).
Otherwise, I do not know what caused the 35012, and found nothing online to describe it. The other strange thing is that if you get the error, Access begins to act really strangly. If you're poking around your database, close and reopen every time you get the error to make sure it is behaving 'normally'.
Hope this helps.
Kevin.