@hongbohe Hi there, thanks for your question. Yes, web connections can be a tricky business at times! We do have a great article available from the FME Community Knowledge page, all about troubleshooting a Sharepoint web connection. These web connections can be particularly tricky, warranting their own troubleshooting guide.
This troubleshooting article also has a couple of Sharepoint-specific guides for creating a web connection which may shed some more light on your particular web connection. See the Related Articles section, at the top right of the linked Troubleshooting article.
I hope these articles help you troubleshoot your Sharepoint web connection. If you're still having authentication errors after some troubleshooting, don't hesitate to reach out again to explore further.