I uninstalled/installed it and it's working now, just have to get use to this wacky layout.
That worked for me as well. And it also suggested another possibility: removing the forum from my list and re-adding it. That worked as well.
So, the answer seems to be that people who are having this problem need only remove the forum from their list and re-add it, then log back in. They can stay on an older version of the software if they want (the version I tried it with is the previous version).