Fixed for me in Xcode 12 beta 5

Did you ever get resolution on this issue? I am experiencing exactly the same issue.

Similar to you both, I also see this error with NSPersistentCloudKitContainer in an existing iOS app which worked fine on iOS 14.5 and Xcode 12. I also notice I am no longer able to use the public database in conjunction with NSPersistentCloudKitContainer, and am forced to use the private database. Perhaps early beta bugs, as @nitramluap said.