I didn't do anything, I assume there was an issue on apple's system here: https://developer.apple.com/forums/thread/789011?page=3 You can see it seems it got solved for everyone at the same time It got fixed for you too, if you click this: https://appleid.apple.com/auth/authorize?client_id=com.smoothmail.signin You will see the issue is now about the request/args, not the client id anymore