is osTicket installed on a Windows server, by chance?
Turn on error reporting and then see what that gives you.
In in /main.inc.php:
#Don't display errors
ini_set('display_errors',0);
ini_set('display_startup_errors',0);
Change this to
error_reporting(E_ALL);
ini_set('display_errors',1);
ini_set('display_startup_errors',1);