pluggable.php fail

Hi

Im getting this error when i try to create an account.

Warning: Cannot modify header information - headers already sent by (output started at /var/www/frisorpriser.dk/jnydesign/2hand/wp-content/themes/twentyeleven/header.php:13) in /var/www/frisorpriser.dk/jnydesign/2hand/wp-includes/pluggable.php on line 680

Warning: Cannot modify header information - headers already sent by (output started at /var/www/frisorpriser.dk/jnydesign/2hand/wp-content/themes/twentyeleven/header.php:13) in /var/www/frisorpriser.dk/jnydesign/2hand/wp-includes/pluggable.php on line 681

Warning: Cannot modify header information - headers already sent by (output started at /var/www/frisorpriser.dk/jnydesign/2hand/wp-content/themes/twentyeleven/header.php:13) in /var/www/frisorpriser.dk/jnydesign/2hand/wp-includes/pluggable.php on line 682