I contacted Apple Support to inquire about this problem (unable to export any files, movies or photos, from the Apple Photo Library (in icloud).
The support person checked some things and could not find anything glaringly amiss. So, he suggested a workaround, which did work. He had me toggle off the "Optimize Mac Storage" setting in the Apple Photos.app's settings.
Of course, I had to wait over 8 hours for the painfully SLOW Apple process to download all the full resolution photos to my Mac Book Pro's internal drive (luckily I had enough spare room to do this).
After that long wait, I again tried to export photos and movies. To my surprise, the tech support's workaround WORKED! Now, I was once again able to successfully export photos and movies at lightning speed.
ASIDE. IMHO, something is terribly wrong with Apple's iCloud service. The downloads of those high-resolution photo files to my internal drive proceeded at a "snail's" pace (averaging only a few kilobits/second!). Meanwhile, running internet speed tests at the same time, showed my down load speeds above 900 megabits/second (as you'd expect for my gigabit service via Comcast). Also, running other applications showed that my Internet speed was fast. Yet, the Apple Photos.app process of downloading thousands of high-resolution photos only moved at a turtle's pace.