Hey fine441! I've finally managed to get it back working with a little workaround!
I'm now successfully back connected to my iCloud account on macOS Sierra as also photos enabled again.
Here's what i did:
First i followed this discussion and got the thoughts on what's the problem here (It's the KeyChain!)
https://discussions.apple.com/thread/7255773?tstart=0
But as on this discussion it wasn't mentioned how the guy from the "Genius Bar" fixed it. I've searched for how to RESET the keychain and then found this:
http://de.ccm.net/faq/3511-mac-os-x-schluesselbund-zuruecksetzen
So, i only followed the tutorial and rebooted my mac. Then i tried it again to login to my icloud and....
HEY, It worked again! 🙂
Then, just set up your settings again and you're back. Also if you've saved your keychain to iCloud before, the newly created one will get back all your keychain items from iCloud right after.
That's it.
Happy testing the new betas and best regards!
Chris