I suddenly have this issue too. I just installed the expansion Get Together and the bug appeared. First there was the "infidelity in front of the loved one with no consequence" bug and now this. I am becoming tired of paying for a game that has bugs nobody at EA cares enough about to solve.
A few suggestions.
1. Get your reputation to Great. The date meter will start with the first section filled in if you have a Great reputation, so the worst you can get is a bronze instead of a failure.
2. Don't bother dating Sims who are known to have this problem. This includes all the DU professors and the owner of The Gnome's Arms (the pub in Henford-on-Bagley). There could also be others I just haven't dated yet.
3. Based on this thread, daytime dates are probably not a good idea if either Sim is a vampire.
