I had the exact same thing happen to me. I tried signing out of iCloud and now I'm unable to sign-in to iCloud.
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!)
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:
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.
Happy testing the new betas and best regards!
Mine is working fine and my drive is encrypted too, so that should rule that out.
Was this a clean install? I already had iCloud Photos enabled when I upgraded my rMBP from El Capitan so it might be issues with a first run process in Sierra.
Can confirm same issue, but I cannot sign in to iCloud as well.
I have exactly the same bug on my german macbook air.......opened a topic about it one hour ago
Same Issue Macbook Pro Retina
Same issue with Photos: unable to activate my Photostream. Other iCloud items work well.