This is all I did

create/add this line to an .htaccess file and place it in your Nuke root directory:
PHP_FLAG output_buffering On

Funny though how you are the only one that has reported this so far and you also see it on your site.
Do you see it on every Nuke6.5 site with Forums?
http://www.nukecops.com
http://www.fauxpas.org
http://www.portedmods.com
Try those sites too if you would and let me know if you see the same error.

mikem