all is not lost, heres a workaround for a problem that shouldn't exist if EA actually play-tested their games.
1. in the new neighborhood (I forget the name) go into edit town mode.
2. you may notice a grey empty lot somewhere, the one in my game was down next the Plumbob Pictures Backlot. Bulldoze the lot.
you should now be able to play without a problem, however if you can't try moving/bulldozing any Ambitions lots.
Edit: Screenshot to show where the lot is in my game (picture quality is a kinda poor because I had the game in windowed mode at the time).