Pete tried this, it may work for you...
1) go into File Menu>File Tools>Manage Pictures and Movies
2) take note of the current picture path for where your pictures draw from
3) set a new picture path (this will clear your pic references).
4) set the picture path back to what was noted in step 2
5) download a cover that was being problematic - seemed to work for us.
Also tried this...
1) deleted the problematic image (if you were trying to download a higher-res version).
2) go to a different title then returned back to it (which zeros out the pic reference since there's no image anymore)
3) download cover - that worked too for us.
Can anyone reproduce this problem after switching to a different folder/zeroing out their pictures, then switching it back to the regular place and attempting the download with the now-zeroed picture references?