Did you get this resolved? I have a similiar error on checkout.

[FONT=&quot]Warning[/FONT][FONT=&quot]: Cannot modify header information - headers already sent by (output started at /home/mysanbiz/public_html/includes/modules/payment/eselect_api/mpgClasses.php:574) in /home/mysanbiz/public_html/includes/functions/functions_general.php on line 44[/FONT]

Site has been up for a year now and clients just started alerting me of the error. Not sure when it started..