They had this same problem with OFB and NL with some people, didn't they? I thought the OFB patch had something about that in it.
Sorry you had a problem and I hope you have a back-up, maybe you can try reinstalling it and see if that helps.
I've already reinstalled, I'm just waiting for the game to load up. The Neighborhoods folder seems to have been unaffected by this, as the modification date appears to be the last time I played the game.
EDIT: Game has loaded up, no warning message. Checked in on a lot with a home business, it works fine.