OAuth client ID or secret is missing — save in Settings first
Admin started Gmail OAuth consent, but exchanging the authorization code failed because client ID or secret settings are empty (gmail-oauth token exchange).
Why this happens
Connect Google ran before xoauth2_client_id / xoauth2_client_secret were saved. Same credentials family as Gmail OAuth client ID and secret are required (mailer assert/send), but this string is thrown at consent-callback time.
Diagnose and fix
1. Admin → Settings → Email — paste OAuth client ID and secret; Save.
2. Then click Connect Google again and finish consent.
3. Follow Gmail XOAUTH2 for redirect URI and Cloud Console setup.
4. After connect, if send still fails, see Gmail not-authorized siblings.