"Cannot modify header information - headers already sent"

Hey Guys,

To begin, thanks for putting together this business WPMU Dev its really awesome!

Regarding my issue, once I ticked the option "enable incomplete signup" I get this error on the sign up page after you enter registration info:

Warning: Cannot modify header information - headers already sent by (output started at /home/travelab/public_html/academy/wp-content/themes/shoutbox/header.php:2) in /home/travelab/public_html/academy/wp-includes/pluggable.php on line 669

Warning: Cannot modify header information - headers already sent by (output started at /home/travelab/public_html/academy/wp-content/themes/shoutbox/header.php:2) in /home/travelab/public_html/academy/wp-includes/pluggable.php on line 670

Warning: Cannot modify header information - headers already sent by (output started at /home/travelab/public_html/academy/wp-content/themes/shoutbox/header.php:2) in /home/travelab/public_html/academy/wp-includes/pluggable.php on line 671

I've seen this issue posted on here a few times, im guessing it has to do with the theme?

The sign up still goes through, I just get that error that I need to fix.

I am pretty experienced with PHP, and have a background in other programming languages. If someone can point me in the right direction regarding what code I need to remove or replace that would be great!

Best regards,
Gerald