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.