"Cannot send session cache limiter" Error

Hi,

I'm not sure what this error is related to but I'm getting this error

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/webbrief/public_html/wp-content/themes/genesis/lib/structure/header.php:31) in /home/webbrief/public_html/wp-content/plugins/pro-sites/pro-sites.php on line 640

when members go to the the pro site page where they click on a site to upgrade or modify.

I've set pay to blog on the site so that the moment members log in, they are asked to pay.

Any help with this will be very much appreciated.