Advanced Filter: Headers already sent error with Marketpress and Pro3/FullWidth theme

I'm working on a development site.

Marketpress ecommerce
Pro3 theme with Fullwidth child

Was using the jQuery filter without issue. Tried to use the 'advanced filter' feature and then got this error thrown at the top of the page (see attached image):

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/content/81/4368081/html/4spottemplates/vanatkins/wp-content/themes/fullwidth/header.php:2) in /home/content/81/4368081/html/4spottemplates/vanatkins/wp-content/plugins/marketpress/marketpress.php on line 989

Any clue on what is causing this and/or how I can fix it?

Thanks in advance.