Forum Error

On my site SonyPS3News when I go to the forums whenever I post a reply or new topic it comes up with the error but the posts still go through

Warning: Cannot modify header information – headers already sent by (output started at /homepages/27/d97350563/htdocs/nick/forum/bb-plugins/sidebarplugin.php:13) in /homepages/27/d97350563/htdocs/nick/wp-includes/pluggable.php on line 689

To check this out click on the link and on the right of the site under menu click forum.

PLEASE HELP!!!