The edit link appears to be working for me, however, the Post Reply AJAX fails for me with a 500 Internal Server error on the add reply call. Two branches of fixes here:
1. Check your server error logs, hope it tells you an easy fix.
2. Disable add-ons and try to reproduce it. If you can't, then you know its an add-on on. Reenable them one by one until you fix it.