If you find that you are suddenly unable to login to iCloud on Mac then follow this simple step to fix it.
- Navigate to Go > Go to Folder...
- Enter the following /Users/<username>/library/Keychains</username>
(where <username> is </username>your Mac username)
- Create a new folder anywhere on your mac and Drag & Drop all the files & folders within the 'Keychains' folder into it (for backup)
- Restart your Mac
- Log in again to iCloud via System Preferences...
That's it you should now be able to login to iCloud on your Mac once more
Trashing Mac User Keychains also fixes other Mac app login issues
Also fixes issues for login with other logins like being 'Unable to login to Adobe Creative Cloud' or 'Constantly being asked for login on Adobe Creative Cloud'. This one was bugging us for ages as it was stopping us from using the latest versions of Illustrator & Photoshop on Mac' and trashing the Mac Keychains in the user folder fixed that also (for us anyway).