Header already sent

I’m getting this error message.

http://managementplatform.nl

Warning: Cannot modify header information - headers already sent by (output started at /home/stefnitert/domains/managementplatform.nl/public_html/wp-content/themes/bp-social/header.php:2) in /home/stefnitert/domains/managementplatform.nl/public_html/wp-includes/pluggable.php on line 868

I’ve googled it but I can’t find spaces in the php tags.

managementplatform.nl/wp-signup.php

Does anybody know what I’m missing here?