Prestashop 1.6 shows blank page

Hello,

I'm having a problem with my prestashop instalation, since it's given me a blank page, whatever the page I go (be it the front page, or my admin folder). I posted a question in another post, where I was told to change the PHP version. I asked the support to do this, since I can't do it), and they told me it wasn't the PHP version, and to make another post. I activated the debug mode, and got the following errors:
Warning: Cannot modify header information - headers already sent by (output started at /home/websto9/public_html/config/defines.inc.php:1) in/home/websto9/public_html/classes/controller/FrontController.php on line 678
Warning: Cannot modify header information - headers already sent by (output started at /home/websto9/public_html/config/defines.inc.php:1) in/home/websto9/public_html/classes/controller/FrontController.php on line 679
Warning: Cannot modify header information - headers already sent by (output started at /home/websto9/public_html/config/defines.inc.php:1) in /home/websto9/public_html/classes/Tools.php on line 134.
Thank you in advance.