Errors when adding item to cart

Any idea what is causing the following errors when adding a product to the shopping cart? I've removed all plugins except domain mapping and supporter and the error is still occurring. I've updated MarketPress, supporter & domain mapping to the latest versions.

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /home/fowi/public_html/index.php(1) : eval()'d code:37) in /home/fowi/public_html/wp-content/plugins/marketpress/marketpress.php on line 742

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/fowi/public_html/index.php(1) : eval()'d code:37) in /home/fowi/public_html/wp-content/plugins/marketpress/marketpress.php on line 742

Warning: Cannot modify header information - headers already sent by (output started at /home/fowi/public_html/index.php(1) : eval()'d code:37) in /home/fowi/public_html/wp-content/plugins/marketpress/marketpress.php on line 1993