I checked getReplyForm() and replyform() in the forum.php to see if permissions are checked correctly.
All fine. After that I realized there must be something wrong with the template which is a modified one
based on blackcandy.
Adding a simple
<div class="clear"><!-- --></div>
before the Forumheader line in ForumReply.ss
<% include ForumHeader %>
helped to create a correct output of $Content in Page.ss
I know this is an old post but I'm having the same problem myself & have tried making the above edit to my code, but still having no joy. I could REALLY do with some help with this as it's one of the final parts of my site I have left to do - typically it's one of the most important areas too!!