David Kimura PRO said over 2 years ago on Sign in with Apple :
It's pretty hard to tell   . During the discovery of creating this episode, I got that kind of message a few times, but nothing in the error pointed me in the right direction to resolve it. I searched the Apple developer forums a few times and found several hints to resolve the issue and one of them finally worked. However, the kind of error message I got was with the handshake and it prevented authenticating the user. I haven't seen this particular case where you are authenticated, but still get that flash message.