We've been trying to reproduce that bug since yesterday; thank you for sharing the thread with support! An interim fix has been deployed.
Some of the branch pickers in affected threads are going to erroneously show < 0 / 2 > on load. They'll start working again after you start using them. You should be able to get to all the messages in the thread.
(The < 0 / 2 > bug has been with us for some time. We tried to fix it earlier this week, but it turns out we only fixed some of the cases and the remaining failures became loud. I'm going to go work on a more comprehensive fix, but, y'know, very carefully.)