Same issue for me, if I set up a second device to work with a public database only, I get UserInfo={NSLocalizedFailureReason=Request 'A72DA6B1-AFA0-4BC1-A116-518B36E9709C' was aborted because the mirroring delegate never successfully initialized due to error: <CKError 0x155c4230: "Partial Failure" (2/1011); "Failed to modify some record zones"; uuid = FDFC2AB5-2552-4A27-AF85-980AF61942CA; container ID = "XXXXXXX"; partial errors: { com.apple.coredata.cloudkit.zone:defaultOwner = <CKError 0x157e8360: "Server Rejected Request" (15/2027); server message = "Custom zones are not allowed in public DB"; op = D361CF9F4B166EE8; uuid = FDFC2AB5-2552-4A27-AF85-980AF61942CA> ..on the first device were the project was developed with, everything works just fine. I am only writing on the _defaultZone, meaning I did not set up a custom Zone as stated by the error messsage. any idea? Thank you!

I would also be interested in the answer