Hey Guys! 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...
Here's what i did:
First i followed this discussion and get 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 on ccm.net 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 to the game... Also if you've shared/saved your keychain to iCloud before, the new created one will get back all your keychain items from iCloud right after! ;-)
So that's it....
Happy testing the new betas and best regards!
Chris