The problem happens because Sony does not embed a full-sized JPG into their raw ARW file.
So when you make a Photo Mechanic crop on the raw and then perform a Save As JPG,
Photo Mechanic applies the crop to the embedded JPG -- the only thing it can do.
But if the embedded JPG is the wrong size, then the crop and subsequent save fails.
NOT Photo Mechanic's fault!!!
Bad Sony for this weeny embedded JPG.