I had the same issue with iCloud Drive feature on my new MacBook Air laptop (M1, 2020), macOS Big Sur, Version 11.2.3. iCloud Drive on the laptop didn’t sync files with iCloud Drive on the iCloud.com (web). I’ve got into this situation after my old MacBook Air laptop (2013) was damaged by the water spill and cannot be accessed any more. When the new laptop (M1, 2020) has arrived it was activated with the same credentials (Admin UID/PWD and AppleID/AppleID PWD) as the user on the 2013 laptop. iCloud Drive feature on (M1, 2020) laptop worked fine for Contacts, e-mails, Notes, Photos, iMessages, etc., but it didn’t work for the iCloud Drive files. The complete new MacOS re-installation on (M1, 2020) didn’t help. The consecutive upgrades from Version 11.2.1 to 11.2.2 to 11.2.3 didn’t help either. I created a new Admin User and new AppleID account on the (M1,2020) laptop and iCloud Drive feature started to working as designed. However, the prospect of moving the files, Contacts, Photos, etc. from the original account to the new one was not encouraging. In addition, I would have to re-assign my iPhone and iPad registered with the original account to the new account and it would require re-installation of all none-default apps on those devices. So, I tried another scenario to fix iCloud Drive synchronization feature. I have changed the Admin PWD and AppleID PWD for the original user on (M1,2020) laptop. As a result the iCloud Drive feature immediately started to working as designed.
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 https://discussions.apple.com/thread/252081633.
[Personal Information Edited by Moderator]
