iCloud Photo Photo Library - Initial Sync keeps resetting, jumps between 3k and 30k photos

Since Photos became available I upgraded iCloud to 200GB and uploaded all photos from my mac. 383 Photos are stuck on "uploading" no matter how long I let it upload (10 Mbit Upload; network conditioner "trick" enabled or disabled).

When I turn on iCloud Photo Library on my iPhone it keeps "preparing" and "downloading" photos. It usually does so for max. 30.000 of 40.000 photos then arbitrarily keeps jumping between 3.000 and 30.000 photos.

If one opens Photos in the iPhone one can even see photos appearing and disappearing in seconds. Even while picking photos via "Moments" from other apps. The process keeps on for days. Oldest photos shown are from 2005, library dates back to 1997.

Mac: 10.10.3 rMBP15(2013), set to keep originals

iOS: 8.3 (iPhone 6 64GB), set to "optimize storage)

Workarounds tried so far:

- Checking Photos library on Mac for missing files (fixed some)

- Repairing Photos library via Option-CMD mode

- deactivating Mac iCloud Photo Library and turning it on again

- Resetting iPhone iCloud photo library via turn off/on

- iCloud Photo Library on iPad Air (1st Gen., 32GB) and iPad mini (1st Gen., 16 GB) which ended in same results as iPhone

My suspicion is, that the 383 photos stuck in Uploading on my Mac keep making trouble. iCloud.com Photos works, Photos on Mac works except for stuck files.

Is there a way (log file, ...) to find out which files are stuck, as I did try visually comparing the librarys on icloud.com and Mac but with the ammount it is hard to see a difference. Btw: please refrain from saying a 150GB library with 40k files is not supposed to be in iCloud or something similar. I also got it in Lightroom but want to make iCloud Photo Library work. 😉

Regards!