Many people had such issue: https://discussions.apple.com/message/29295728?ac_cid=tw123456#29295728
I notice when upgraded to 10.11.1, I cannot login any iCloud nor iMessage/faceTime account on my old and sole admin account.
I tried to delete ~/Library/Application Support/iCloud/accounts, no luck
I tried to erase NVRAM, no luck
I tried safe mode, no luck
I tried to reset key chain, no luck
I managed to rerun setup assitant and create a new admin account, now I can login to my iCloud.
iCloud always says You can't sign in at this time, try signing in again
iMessage will pop a window ask me again to enter password after I entered it when entering the account name and password, and then says Verification Failed, An unknown error occurred.
So.. I am desperate, and I think it is supposed to be some broken files for my old admin account, but I don't know what files to be deleted to let OS X to create new files to restore the state.