Forum Error aftter Upgrade
 

I upgraded to 4.0 and all went well until I got past the last step to get into my Admin CP I received the error message

Unable to add cookies, header already sent.
File: /home/disneyfo/public_html/forums/includes/class_core.php
Line: 4372

I disabled the plugins via the config.php file and I was able to get into the admincp to disable the plugins and all then went well with the forum but if I turn the plug in system back on the same error appears. Is there a way to figure what is causing it