Zen Cart will hide the PHP error output at includes/application_top.php

if (defined('STRICT_ERROR_REPORTING') && STRICT_ERROR_REPORTING == true) {
@ini_set('display_errors', '1');
error_reporting(E_ALL);
} else {
error_reporting(0);
}


To attempt to got the PHP error output, create the includes/local/configure.php:

define('STRICT_ERROR_REPORTING', true);

Then try to access the site again. You may see the error output.