OK, commenting on your thread as I have the same issue (which you've found). Let's hope Apple is able to track down what's going on here. Only started with iOS15 beta 4 from my testing. https://developer.apple.com/forums/thread/687877

This is NOT fixed with beta 5. Two problems related to NSPersistentCloudkitContainer persist since beta 4. I've submitted a bug report to Apple - no response yet. The issues: "Account temporarily unavailable due to bad or missing auth token" when using the Development Environment (device or simulator), despite being signed in and despite it working on beta 1, 2 & 3. When using the App Store version of my App (in the Production Environment) on an iOS 15 device (beta 4 & 5), the syncing appears to start but nothing syncs. The app crashes to the springboard after a minute without warning. Crash reports are vague but I have captured a CloudKit sysdiagnose and sent it to Apple in the hope they can decipher it. I really hope they sort this out as it's going to render my currently available app useless if iOS15 is released in this state and my users install it.

Seems like a new issue in Beta 4 resulting in this error: "Account Temporarily Unavailable" (1028/2011); "Account temporarily unavailable due to bad or missing auth token" 'Reset All Contents & Settings' on the simulator doesn't appear to fix it this time. I've also noticed that the current App Store release version of my App which uses NSPersistentCloudKitContainer does not sync properly and crashes after 30 seconds. Nothing has changed other than installing Beta 4. I've submitted a bug report to Apple.

Hopefully we'll find out next week when beta 2 drops! 🤞🏼😬

I'm having the same issue with an established app with Xcode 12 & iOS14 using NSPersistentCloudKitContainer. When I try to run the exact code with iOS15 & Xcode 13 (Developer Beta 1 for both) I see the "Failed to sync user keys" error. I suspect this is an early beta bug so hopefully it will disappear in subsequent betas.