Okay, I've updated the forums to the latest version of vBulletin, but there seem to be a few minor bugs to work out. For now, you can't open a thread by clicking on the thread title in the main forums index. You have to open that section first and click on the thread from inside that section. This seems to go away once a section receives a new post, so it should only last as long as it takes someone to post in every new section.
At least the error messages following every post seem to be gone.