Auth0 OAuth returns 500 Internal Server Error for specific user

Environment

• Package: @account-kit/signer (latest)
• Auth0 Connection: Username-Password-Authentication (database connection)
• Network: Sepolia

Description

When using AlchemyWebSigner with Auth0 OAuth, the /api/signer/oauth endpoint returns a 500 Internal Server Error for a specific user authenticating with Auth0's database connection (email/password).

Other users (including other email/password users and social login users) work correctly with the same configuration.

Error Details

POST https://manage.g.alchemy.com/api/signer/oauth
Status: 500 Internal Server Error

Request payload:

{
  "code": "ROmRasHNxnw0hZMS-...",
  "state": {
    "authProviderId": "auth0",
    "isCustomProvider": false,
    "turnkeyPublicKey": "041a15be4b5503d8cdb43825197c9ddc1116697b2c6be47d3c0f24d14cade13dd8329c353ab8461df0209a8d48d77b44dc1bc60a3df191f200642e99759ad07602",
    "expirationSeconds": 900,
    "redirectUrl": "https://localhost:8081/#/wallet"
  },
  "redirectUri": "https://signer.alchemy.com/callback"
}

Expected Behavior

The OAuth flow should complete successfully.

Additional Context

This suggests possible corrupted state in the MPC system for this user. Happy to provide the user ID privately if that helps debug.

Begin by reproducing the failure in the @account-kit/signer AlchemyWebSigner Auth0 database-connection flow and inspect the /api/signer/oauth request and response. Compare the affected user's result with working email/password and social-login users; the issue is done when the affected user's OAuth flow completes successfully without a 500 response.