Sierra Upgrade

Hi, I've upgraded my MacBook Pro from OSX El Capitan to Sierra but after logging in to my MacBook Pro 13, iCloud will not accept my password, even though it was accepted during the setup phase of the upgrade. My other devices use the same password and work correctly. I suspect the problem is to do with the iCloud Keychain. I read elsewhere that there may be a
fix on this site although I haven't found it yet. A second problem concerns the Mail App: a window appears called "Mail Upgrade" which can only be removed by right clicking the mail App and selecting "Quit", so I have no email either. I've re-installed Sierra but the problems are still there. Any pointers would be greatly appreciated.