corrupted library (?) preventing ipad upload to icloud photos
I can only tell you that I have the same problem, but on my iPhone. The only time I get it to work is when I turn iCloud entirely on and off. Yet – after a successful sync with all images and videos everywhere, the sync problem reemerge. Your mileage may vary. Try a complete iCloud off/on (and – I would recommend – a reboot in between) and see what happens.