Sorry to use the answer button but i have the same problem, my photos on my iMac stop to upload to the Cloud when around 20% is done. I had this problem before so i restarted the photo and deleted all photos so nothing was left in the cloud and in photos on any of my devices.
I uploaded all photos again in photos on my iMac and activated icloud again on my devices, and the same result. The uploading stops after 20% of the photos are uploaded and nothing happens. I have more than 90% left on my account for icloud so no prblem there. How to kick start the uploading again?
If this won't get solved soon i need to find another solution for my photo library and I do not want that I want to use the features as they are supposed to be used.
Please anyone, how can i do this? And if the solution is to stop the icloud on all devises and then start it again i have tried that and to only result was that the photos already uploaded was uploaded again so i ended up with two of every photo before the uploading stopped again at the same place as last.
Desperate Björn.....