I've experienced this Jet error on my Embrace fansite, Happy and Lost, as well as on our development server at work.
At work I was creating a new database object every time I needed to do a DB call - not clever. So I added a check which said:
if (db == null) db.Create();
Or code to that effect, anyway. Worked like a charm but I don't know why. It must be something to do with concurrent connections, but I never saw any decent solution online so this may help someone.