It sounds like some problem with redirecting. When you post in the forums its on ForumPostEdit.aspx then after posting it redirects to ForumThreadView.aspx. So the post does get posted right? You just get this error after posting?
I will see if I can replicate the problem. Any additional info you can provide may help.
Does changing the skin have any effect on the problem?
Are you using ssl on your forums page?
Best,
Joe