My recommendation for the Apple Customer Support (I worked with two Senior Apple Support Advisers) would be to try to changing Admin PWD and AppleID PWD for all scenarios when iCloud Drive sync feature is not working. For the Apple Software Engineering I would recommend to take a look at the new laptop activation scenarios when the user is entering the same credentials for the new laptop as they were established for the old laptop on the same account. The Migration Assistant (Finder->Application->Utilities->Migration Assistant) functionality might be also affected. Thanks for the hunch to DiZoE user from this discussion board