Hey zlax just a question did you search http://www.phpbb.com for this because most of these errors (or whatever you are calling this) are mentioned there.
I am sorry if this is caused by any of the mods that came from my site.
Sorry i wish i could help you out some more but this is the first time i see this.(maybe just a line that is misplaced in your viewforum.php code or your viewforum_body.tpl)