so you've tried to Share from motion strainght to proRes?
I guess i would create a copy of the file and keep deleting things till it worked and see if that item had a problem. Stills can be a problem, i've had issues with jpegs and native psd files, that open and play fine but then corrupt at export. Often simply opening them in some app and resaving is enough to get rid of the problem. Or save as pngs and reconnect the media within Motion.
Did you clear the RAM cache before exporting, i think this could cause problems.