I have had a few people contact me about not being able to add new posts into the forum.
I realise it is quite frustrating - and I have spent a lot of time trying to track it down. I've sniffed responses, traced Sql, turned google inside out and so far have come up with the following resolution:
"It's a bug in the Forum Modules related to notifications"
From what I can glean, if you have subscribed to notifications from a particular forum or thread, that can cause the error. So, what I suggest is that you either unsubscribe (if subscribed) from a particular forum/thread before trying to create a new thread. Or try subscribing, I've had success both ways.
I'll wait until the new DNN forum (4.4.4) release and update the module as soon as I can. Until then, be patient and you can always either PM me or use the enquiries form ('Talk to Us') to request help.
There has been another problem with notifications, in that some key SMTP details were incorrect, so sometimes people got notified of replies, sometimes not. This should be fixed now, up until the point where the DNN Forums notification bug kicks in.
For more information about the DNN Forums bug, see this thread on dotnetnuke.com www.dotnetnuke.com/Community/Forums/tabid/795/forumid/7/threadid/203003/scope/posts/threadpage/1/Default.aspx
(don't you think the DNN Forums could do with a bit of Friendly Url Action??)