I am having a problem with regard to customers logging in and I can not seem to find the problem.

Customers can create accounts, but when they go to log in all they get is a blank page. Interestingly enough, if after attempting to login and the blank page displays, you can visit https://mysite and it appears that you are logged in.(shows my account and logoff items)

But, when you go to add items to you cart another blank page is displayed....

Site: http://www.piercingsun.com/

I am seeing some of the following in the error logs:

PHP Warning: Cannot modify header information - headers already sent by (output started at .../html/includes/languages/english/header.php:2) in .../html/includes/init_includes/init_templates.php on line 78
PHP Warning: Cannot modify header information - headers already sent by (output started at .../html/includes/languages/english/header.php:2) in .../html/includes/functions/functions_general.php on line 44