TrustFailure error with a WebClient on OVH server in Xamarin

I have 3 environments, 1 for dev, 1 for tests, and 1 for production.

The 3 run the same code, but dev and test are on our web server (Gandi), and prod on client web server (OVH).

The server is used for some web services, and was running great for months. The issue appeared on the 26 of September.

I am using a WebClient object in Xamarin to call the web services.

No issue on dev and test, but "Error : TrustFailure (Authentication failed, see inner exception)" on prod.

I made a lot of research, and it seam to be an SSL certificate issue. On my OVH panel, the SSL certificate are enabled and the web site is working great with HTTPS.

I am a newbie with network / server issues, and running out of idea. My htaccess seam to be ok.

Thanks