To troubleshoot, you need to tell your webserver to log PHP errors. Here's how you can get started. Just put this in your .htaccess file then be sure you make a "/logs" folder or some other folder and update the path to accommodate your browser.
For this to work, you will need to edit…
# enable PHP error logging
php_flag log_errors on
php_value error_log /home/path/public_html/domain/logs/PHP_errors.log