Ah, thanks Gavin and Paul!
Gavin, it is WBWK 2.8 and pg 2.8.1. It happened for all nine problems in a particular quiz that had inequality problems. Since the instructor has all nine problems on one page, I think it is quite likely that Paul has it, and that the students were spending over 30 minutes finding answers before hitting submit.
Paul, I have experienced the same thing while editing, and it is annoying. I copy the whole problem to a different editor where I can mass search-and-replace all of the special characters. But I had not associated this with logging out due to inactivity. I'll bump up the logout time for the course and hopefully that will fix this for now.