I just installed the silverstripe-v2.2.2-rc3.tar.gz and have installed the forum and the youtube gallery. The forum registration doesn't work, and neither does creating a new topic. (I saw another post from Sigurd saying they were aware of the registration problem.)
The YouTube gallery installs fine and you can add one in the CMS, but it comes up with a blank page when looking at the published version. The same sort of behavior as the registration on the forum.
Are the modules not compatible with 2.2.2-rc3?