[kh99]

That message is the phrase 'forumclosed' and the code in newthread.php is this:

if (!$foruminfo['allowposting'] OR $foruminfo['link'] OR !$foruminfo['cancontainthreads'])
{
	eval(standard_error(fetch_error('forumclosed')));
}

so if we assume that you have posting permission and the forum can contain threads, that leaves $foruminfo['link']. Did you enter something in forum's "Forum Link" field? (although if you did, that should redirect to the link instead of showng you the forum).