I just ran into exactly the same problem and I don't know how to solve it!
yakubenko_m
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I just ran into exactly the same problem and I don't know how to solve it! The problem persists even on my iPad Pro, where the maximum version is 17.7.3, which means you can't upgrade to 18!
The TestFlight 3.7.1 update has been released, but the error has not disappeared. I tried: deleting the app, reboot the device, log out and log in from the account, reset the network settings – nothing helped! But as soon as I log into another App Store account on the same device, which my application testing colleague gave me, everything worked. Does it mean that Apple restricts my Apple Account to TestFlight???