This issue seems to happen exclusively to Macs running macOS Monterey beta 4. We did some preliminary investigation and believe it's a macOS Monterey bug.
To resolve this immediately, please update to macOS Monterey beta 5 (21A5304g). Alternatively, you can downgrade your macOS version.
In general, we recommend being careful when installing beta versions of apps or new operating systems as they may break the functionality you rely on.
If you're affected even though you're not on macOS Monterey, please try these suggestions for iCloud sync issues, and if they didn't help, send us Diagnostic Data on all your devices.