Double check the screw to make sure it isn’t touching the pup wire in the pickup cavity. If it’s not and you suspect it’s grounding out inside the can you could try a hillbilly fix - the screw is a slip fit through the pup can, slip a piece of shrink fit tubing over the screw threads (that’s long enough to poke through the pup back plate) shrink it with a hair blower and reinstall the pup and screw to see if that provides adequate insulation. If you don’t have heat shrink tube you might get by with a tight wrap or two of electrical tape around the threads. Obviously leave enough threads exposed to bite into the wood.