Warning error with Network theme and pro sites

After over a week of headaches, I decided to abandon use of my favorite theme, Thesis, on my multisite. Killed me, because I love Thesis, but no one here OR at DIYThemes could help me. Finally, I decided to switch to a WPMU theme, because I need compatibility with Pro Sites. Looked through the files to find one that suits the site. Found one that will do: Network.

Installed and activated it today and -- ERROR. Shows in the header on the home page only (kind of hard to see because the text is the same gray as the header -- except the funcion.sessions-start link).

OK, guys, I'm using YOUR theme and YOUR plugin and I'm still getting this error . I get it when there are NO plugins except Pro Sites running.

PLEASE HELP. I have a workshop next WEDNESDAY that I have put off opening so I can get this fixed. I've wasted so much time trying to solve this and now we know it is NOT a Thesis problem. I think the DIY folks were right, it looks like a PRO SITE PROBLEM. Please help me resolve this ASAP.

My site is here PopCred.

Thank you for your help.

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/popcred/public_html/wp-content/themes/network/header.php:2) in /home/popcred/public_html/wp-content/plugins/pro-sites/pro-sites.php on line 639