Hey there! Hope this can help!
In my experience, it’s most likely due to a keychain conflict,
If you sign into another admin account, (even for testing), sign into iCloud is there any issues?
If not, most likely your local keychain needs to be reset:
https://support.apple.com/en-us/HT201609
This WILL wipe the passwords stored on your computer, so having a backup is a good idea, and if you already use iCloud Keychain, and signed in on another Apple device, you can easily sync the passwords again by signing in.
Or you can document them first somewhere if need be.
In short:
Open Finder > Applications > Utilities > Keychain Access.
From the top menu, hit Keychain Access > Preferences > Reset Default Keychain, then restart and sign into iCloud again.
This might not be the solution, but, in my experience most likely is, otherwise I may have a different suggestions, (Such as repairing the permissions of your user account, clearing the certificate cache or IDS, although more isolation would be needed to determine any next steps). Hope this helps!