The system admin has turned off OAuth2 Service Provider?

Summary

error message → the system admin has turned off OAuth2 Service Provider.

Steps to reproduce

reverse proxy config : IIS with URL Rewrite as a reverse proxy - part 2 – dealing with 500.52 status codes | Microsoft Learn

ver : Team Edition 5.12.0

Expected behavior

reverse proxy ?

Observed behavior

public PC → GitLab web direct connection login OK

public PC → mattermost gitlab SSO login error

private PC-> no trouble

why error message?? → the system admin has turned off OAuth2 Service Provider ??

help me ~~