We having the same issue with a few of our test devices as well as reports of some of end users experiencing it. Interestingly those test devices used to generate valid DeviceCheck tokens 1 week ago but not anymore and been in that state for the whole week now.
Did anyone work out how to fix this other than factory resetting the device? In case this is helpful to anyone here are some more details:
verified that DeviceCheck token is submitted from our backend to Apple is identical (byte by byte) to one obtained from iOS by our app
verified that each time DeviceCheck token is obtained from iOS they are different. Seems to be same number of bytes though.
DeviceCheck token that Apple says is invalid seems to be a bit shorter (2190 bytes) than one that it says OK (2236 bytes).
verified that we are using a different transaction ID each time we submit the DeviceCheck token to Apple.
force restarting the phone doesn't seem to fix the issue.
updating iOS to the latest version does not seem to fix this issue
bchoi
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We had opened a ticket with Apple support about this and one thing they suggested was to check if the device's clock was in sync with a well known time server i.e. "Set Automatically" option is turned on in "Settings -> Date & Time". This didn't help in our case (since our test devices were already n sync) but posting it in case it would be helpful to others stumbling upon this thread.
FYI. Apple has indicated this is now fixed in iOS 15 beta 6.