The initial migration from iPhoto to Photos was properly done on my Mac. After a few days my entire library was also available on https://www.icloud.com/#photos. Everything was working fine. Then, following one of the last 10.10.3 beta update, Photos app is out of sync with iCloud. In the log files I can see that ‘cloudphotosd’ crashes all the time. And in the Photos app, the “activity” item in the left lateral bar quickly appears and disappears. It does it again and again. In addition https://www.icloud.com/#photos is now indicating that photos are being prepared. More that 6 days now….
I tried to repair the Photos Libray, it does not change anything. I reported back to Apple.