I have this issue as well. Just migrated from a shared hosting environment to a VPS. This is the only change I’ve made. My stats:
Wordpress 3.1
Buddypress 1.2.8
Default Favicons
Root Directory
Core files untouched
Plugins:
BuddyPress Template Pack
W3 Total Cache with Object Cache & Minify active
BP-Album+
MarkItUp RTE (manual JS plugin install)
So the issue is only on some groups. On both groups that work properly and don’t, there are no obviously problematic HTML validation errors. On the groups that are problematic, MarkItUp doesn’t appear on the textarea, and when “reply” is clicked, the page reloads (hashtag visible), and no comment box appears. Oddness. Thoughts?