going to come down to javascript conflict.. see: http://codex.simple-press.com/codex/faq/troubleshooting/what-is-this-jquery-conflict/
in this case, it might be because you are running a minify plugin which seems to be corrupting js… since the forum js is already minified, doing it again can lead to conflicts…
additionally, it appears you are caching the forum… you cannot cache the forum since every page is dynamic (short of internal caching that we use)… you need to tell your caching plugin to not cache the wp page the forum is showing on…