It's an AJAX issue when you try to quick edit (or quick reply), it will just hang and not do anything.
I used to think it was because my board was heavily modified and that some mod was breaking the AJAX, however my 2nd board has no modifications and it does the same thing.
It also happens intermittently so you can imagine how long it took me to realise what my members were reporting as bugs cuz it never actually happened to me... Til now that is!
Both vb.com and vb.org say that it's an AJAX issue and turn off AJAX is the only fix.