I'm trying to review whether my database files are actually populating to the CloudKit private database. For Private and Shared databases, I get a message that I need to sign in again. I do sign in again, but this message never goes away. I have also tried acting as iCloud User. When I enter my iCloud user id and password, I get an authentication error.
CloudKit won't sign in for Private Database
fwiw I need to disable Advanced Data Protection and and enable iCloud.com in order to log into the iCloud developer console. maybe you're hitting the same?