Tarnation.
I had two pages of unread posts. Page 1 would list fine, but Page 2 wouldn't come up. I was reading new posts okay for a while, but eventually I hit an unread post that didn't want to list properly, and now Page 1 won't come up either.
It seems like maybe there's a certain problem post(or maybe more than one) which is causing the error, rather than a forum-wide bug. Based on the "he who smelt it, dealt it" technique of investigation, I blame Shecky.