I had/am having a very similar problem. My first singer on her first gig was set to perform at the coffee shop. I set up the stage and it sounds like the same thing happened to her as it did Widonja's Sim. She froze in place and didn't move (the occasional twitch here and there, however). I was already set to separate the household anyways after this performance, so I moved her sister to a new house. After playing it for many Sim days (possibly two Sim weeks) the singer is STILL frozen on the stage at the coffee shop!! I didn't even notice until I happened to be strolling by.
Now, on top of that, I'm not sure if this bug is related, but her sister and her new born baby AND her cat AND dog stopped aging! And her banana tree stopped growing too.
AND there's more! Relationship meters are stuck as well. It's like they're all frozen! Everything! The baby even says it has 6 days until it grows up, but it's been at least 7 since he was born and according to my aging options the baby stage should only last 3 days.
Seriously.