Getting error "Failed user key sync" when trying to connect to CloudKit after Xcode 16.1 update / iOS 18.1

Hey there,

I’m feeling pretty desperate at this point, as my most recent update to Xcode 16.1 and the new 18.1 simulators has basically made it impossible for me to work on my apps.

The same app and same code run fine in the 18.0 simulators with the same iCloud account logged in.

I’ve tried multiple simulators with the same results, even on different computers. I’ve also tried logging in repeatedly without any luck. The CloudKit database logs don’t show any errors or suspicious entries. Reinstalling the app on the simulator doesn't help either.

Whenever I launch the application in Xcode, I'm getting:

error: CoreData+CloudKit: -[NSCloudKitMirroringDelegate _performSetupRequest:]_block_invoke(1240): <NSCloudKitMirroringDelegate: 0x600003d213b0>: Failed to set up CloudKit integration for store: <NSSQLCore: 0x103f124e0> (URL: file:///Users/kerstenbroich/Library/Developer/CoreSimulator/Devices/57BC78CE-DB2A-4AC0-9D7A-43C386305F56/data/Containers/Data/Application/EFDE9B05-0584-47C5-80AE-F2FF5994860C/Library/Application%20Support/Model.sqlite)
<CKError 0x600000d3dfe0: "Partial Failure" (2/1011); "Failed to modify some record zones"; partial errors: {
	com.apple.coredata.cloudkit.zone:__defaultOwner__ = <CKError 0x600000d7c090: "Internal Error" (1/5000); "Failed user key sync">
}>
error: CoreData+CloudKit: -[NSCloudKitMirroringDelegate recoverFromError:](2310): <NSCloudKitMirroringDelegate: 0x600003d213b0> - Attempting recovery from error: <CKError 0x600000d3dfe0: "Partial Failure" (2/1011); "Failed to modify some record zones"; partial errors: {
	com.apple.coredata.cloudkit.zone:__defaultOwner__ = <CKError 0x600000d7c090: "Internal Error" (1/5000); "Failed user key sync">
}>
error: CoreData+CloudKit: -[NSCloudKitMirroringDelegate _recoverFromPartialError:forStore:inMonitor:]_block_invoke(2773): <NSCloudKitMirroringDelegate: 0x600003d213b0>: Found unknown error as part of a partial failure: <CKError 0x600000d7c090: "Internal Error" (1/5000); "Failed user key sync">
error: CoreData+CloudKit: -[NSCloudKitMirroringDelegate _recoverFromPartialError:forStore:inMonitor:](2820): <NSCloudKitMirroringDelegate: 0x600003d213b0>: Error recovery failed because the following fatal errors were found: {
    "<CKRecordZoneID: 0x600000d62340; zoneName=com.apple.coredata.cloudkit.zone, ownerName=__defaultOwner__>" = "<CKError 0x600000d7c090: \"Internal Error\" (1/5000); \"Failed user key sync\">";
}

Any help/ideas would be much appreciated, because I have no clue what to try next.

Thanks a lot!