Source: https://www.prestashop.com/forums/topic/190303-error-500-internal-server-error-on-all-pages-except-for-admin-front-page/?p=1533337
if you’ve got internal server error = turn on error reporting – then instead of this message you will see detailed information what and where doesnt work.
open config/defines.inc.php
find this line define(‘_PS_MODE_DEV_’, false);
change to look like this: define(‘_PS_MODE_DEV_’, true);