While it's kinda ghetto, I'd probably start by editing the orders.php file and inserting a bunch of die() statements between the various statements from lines 1-55. Basically to see where the code dies. Put in a die('it got this far'); statement, reload the page in the browser, and see if the message displays. If it does, move the die statement down a line and repeat.

If it croaks before application_top returns, then it's probably something in the init system that's failing. If it croaks when loading and instantiating the orders class then it could be something in there. And so on.