Bug
If I have the Topic Thumbnails theme component enabled, it breaks my forum. I am at 2.9.0.beta4 [222c8d9b6a]. I am using the graceful theme.
Disabling the Topic Thumbnails theme component unbreaks my forum, but of course now topic thumbnails do not appear.
I created this bug post but someone recommended I post here since the bug is not with the forum itself.