Sometimes it seems that Apples iCloud sync seems to do not exactly the right thing. During initial upload of my photos I multiple times had the issue, that it was not possible at all to upload the photos.
When looking at the sync pane of the Photos app, it was something like “18 hours remaining…” but nothing happened, even after two days. The system process responsible for managing the photo upload is called
cloudphotosd. Simply killing the process solved the problem for me.
To fix it, quit the Photos app and run on the terminal:
sudo killall cloudphotosd
Afterwards start the Photos app again and uploading should continue as intended.