I have the same issue. Exact same App work under WiFi but just hangs with LTE. I don't relish a full settings reset where I have to re-add my CC's to Wallet. Can anyone else confirm the issue and have any other fixes other than waiting for the next iOS update in October?

Actual Error: Context saving error: Error Domain=NSCocoaErrorDomain Code=134040 "Save operation could not be completed." UserInfo={NSAffectedStoresErrorKey=(     "<NSSQLCore: 0x102c2b760> (URL: file:///var/mobile/Containers/Data/Application/1505976C-354D-4B4F-A1DE-E6A312F01E99/Library/Application%20Support/private.sqlite)",         (         "<NSSQLCore: 0x102c2b760> (URL: file:///var/mobile/Containers/Data/Application/1505976C-354D-4B4F-A1DE-E6A312F01E99/Library/Application%20Support/private.sqlite)",         "<NSSQLCore: 0x102c301f0> (URL: file:///var/mobile/Containers/Data/Application/1505976C-354D-4B4F-A1DE-E6A312F01E99/Library/Application%20Support/shared.sqlite)"     ) ), NSUnderlyingException= threw while encoding a value.}

I believe it might have something to do with NSSecureUnarchiveFromDataTransformerName, Core Data and CloudKit with NSPersistentCloudKitContainer. But I don't know why this would be in the unmodified demo from Apple? Any help much appreciated.

I'm having similar issues, here's my forum question. Trouble with WWDC21 sample code for CloudKit

I’m having the same issue. Xcode 13 beta 4 works however.