change server rights crash site

After I try to change the cmod off header_inc_php to 644 , I could't because the server apache has te right. Oke after I change that to the user right, the site is down. 

This error I got:

Warning: Cannot modify header information - headers already sent by (output started at /home/jansen/domains/gastenbergum.nl/public_html/inc/header.inc.php:1) in /home/jansen/domains/gastenbergum.nl/public_html/inc/header.inc.php on line 127

It check the url... but there is nothing changed... I have try to set the right back, not possible...
So How do I get this right?