Hi,
I think the screw that you are refering to is smaller than the others, right? THis is attached to a small clamp which holds the voltage regulator on the Phatbox PCB against the back plate. If you have disturbed it, then once you have the box open, you should check that the clamp is still effective, and re-tighten the screw if needed (you can secure it with some borrowed nail varnish). Also check that the thin rectangular plastic sheet is still held between the regulator and the rear plate, since this is a special thermally conductive electrical insulator. It is important that it is in place.
Regards,
Les.