Sign in with Apple fails with "Sign Up Not Completed" for every app in our team

Team ID: ED9QVUCFPE (individual) Bundle IDs affected: com.lumevolab.justmorriz, com.lumevolab.memiq Broken since: 28 July 2026, immediately after the Apple Developer Program membership was renewed/reactivated. Still broken on 21 August 2026 (24 days). Apple Developer Support case: 20000131208746 (closed: "no anomaly found on the membership", referred to the forums) App Review rejection: submission f087942b, 13 Aug 2026, guideline 2.1(a), "unable to log in with Sign in with Apple", on iPad Air 11" M3 / iPadOS 26.6

Symptom

The native Sign in with Apple sheet opens, Face ID succeeds, and then the sheet shows the red error "Sign Up Not Completed" ("Registrazione non completata") underneath the Continue button. The sheet stays open, so ASAuthorizationController never calls back — the app-side code never runs. The sheet's own wording is "Create an account for <App>", so it is the account-creation step that fails on Apple's side, not our token handling.

Device log (captured with idevicesyslog during a reproduction)

akd  ... signing request, calling GrandSlam (gsService)
akd  ... server response (733 bytes)
akd(AppleIDAuthSupport) setError: 2:M2 missing (bad password)
AuthKitUIService ... Showing Authorization UI Alert     (8 ms later)

M2 missing is the SRP exchange: the server did not return its M2 proof, i.e. it refused the client's M1 for this operation.

Why this is not an account or device problem

• Sign in with Apple works on the same iPhone with the same Apple Account in third-party apps (verified with Vinted).
• It also failed on the App Review team's own iPad, with their own Apple Account (rejection above). So it is not tied to one account or one device.
• It fails identically on two different App IDs of the same team.
• Tried and made no difference: device restart, changing the Apple Account password, signing out and back in to the Apple Account.

Configuration — verified via the App Store Connect API, not by eye

What we tried on 21 August 2026, with no change

1. Accepted the pending Program License Agreement (it was released on 18 Aug, so it cannot explain a failure that started on 28 Jul, but we ruled it out).
2. Removed and re-added the APPLE_ID_AUTH capability on the App ID via the App Store Connect API (DELETE then POST /v1/bundleIdCapabilities), to force Apple to re-provision the server-side Sign in with Apple record for the App ID. Device restarted afterwards. Same error.

Prior art suggesting a server-side cause

• Forums thread 790323 — identical akd … "M2 missing (bad password)", same asymmetry (other apps fine, one team's apps broken), thorough config checks. It resolved by itself after 11 days with no developer action.
• Threads 789588, 789418, 789579, 803591 — same class of report, no client-side cause ever identified.
• June 18–23, 2025: Apple acknowledged a server-side configuration fault producing exactly "Sign Up Not Completed" + invalid_client, affecting only accounts that had recently created or modified App ID / Services ID configurations (thread 790827). Our team's capabilities were re-provisioned at renewal on 28 Jul 2026 — the same class of event.

Ask

Please check the server-side Sign in with Apple relying-party record for team ED9QVUCFPE. Everything under our control is correct and has been re-created from scratch; the failure is in the sign-up call to GrandSlam.