There is something wrong with the mention system. I used a lot of at (@) in one post (referring to refresh rate). It seems it tried to resolve every (at)50 and (at)60 to user names and then died. Quick reply didn't work at all, while advanced mode gave me a blank page (only headers sent, no content at all). Tried to post a dozen of times before I realized what was the problem and changed all the (@) chars to (at).