Has anyone solved this issue yet? Unfortunately Aidano's solution did not work. I also have tested and learned that it is a user-specific issue, and not a device specific issue. I have been in contact with a (very helpful, thank you!) Apple engineer through a code-level support ticket and they said that sometimes testflight user accounts will get corrupted. However, this issue also affected the app review process and was the reason for my app's rejection from the app store, and I doubt they would be using corrupted accounts.