Just a thought, but do you still have EOS Utility open in the background? Just tonight i had something very similar happen to me with my Nikon D5300 (it was the first time i had seen this). As it turned out, i had DigicamControl open in the background, which meant both programs were connected to the camera, which i don't think SGP likes. As soon as i closed DigicamControl my captures were able to download ok.
Worth a try!