Sims won't talk to each other

I just experienced a weird bug:

My Sims were in the park. They played chess, chatted for a while, just business as usual - but suddenly, they stopped interacting with each other. The interactions menu was still available, but when I clicked anything, nothing happened. I also tried to introduce my Sim to a stranger: He stepped up to her, and just stood there without saying anything. All interactions stayed in the queue., while both Sims stood there and did nothing. (Technically, they didn't freeze, their eyes and body were still moving.)
Resetting the Sims didn't solve the problem.
When I sent my Sims home, the problem was gone.

(I use all EPs, GPs, APs, no CC/Mods)


(Sorry for bad english, it's not my first language.)