Blank screen after login

Hello, im new to this forum and to customizing zen-cart templates. After some hours breaking my head and figuring out how to edit the template im finaly getting somehwere. But there's a problem. When i login i get a blank screen ( customer login ) and if i go back in the browser im logged in. When i log out i get a blank screen aswell but when i go back 1 page im logged out again. when i forward after going back it says im logged out but not the first time ! What can be the problem ???

i also run the debug file and this is the outcome : [05-nov-2008 22:55:42] PHP Warning: Cannot modify header information - headers already sent by (output started at /home/fantixn/public_html/webshop/includes/languages/dutch/header.php:1) in /home/fantixn/public_html/webshop/includes/init_includes/init_templates.php on line 78
[05-nov-2008 22:55:42] PHP Warning: session_regenerate_id() [<a href='function.session-regenerate-id'>function.session-regenerate-id</a>]: Cannot regenerate session id - headers already sent in /home/fantixn/public_html/webshop/includes/functions/sessions.php on line 167
[05-nov-2008 22:55:42] PHP Warning: Cannot modify header information - headers already sent by (output started at /home/fantixn/public_html/webshop/includes/languages/dutch/header.php:1) in /home/fantixn/public_html/webshop/includes/functions/functions_general.php on line 44

but i never modified the functions_general.php file and i doubt this is the problem

the blank page is with this URL http://fantix.nl/webshop/index.php?main_page=login&action=process

I encounter another similar problem, when i add articles to my cat's i also get a blank page http://fantix.nl/webshop/index.php?main_page=index&cPath=2

Look at the top of that page. Some weird happenings that show on the first line of the page source.

Generally means you edited a file and missed a , } ) / or something important to php.