"Syncing has been disabled due to an error," no indication of what the error is

A new nightmare. After doing some OS updates, I can no longer see any of my iCloud drive data. When I try to open any folder, I get an error that reads: Syncing has been disabled due to an error and zero items are listed in the folder. No indication of what the error might be. All the settings in Preferences related to iCloud appear normal.

[Re-titled by Moderator]