We are running into a 407 Proxy Authentication Required problem when we try to call a webservice inside a plugin, we tried to modify the server's proxy or to set the default credentials inside C# code but still get the same problem.

using (WebClientEx client = new WebClientEx())


                       /client.Timeout = 60000;
client.Headers.Add(HttpRequestHeader.ContentType, "application/json");
licenseResposeJSON = client.UploadString(Url.ToString(), "1");


The problem only resides on the production envirennement. is it related to a firewall or some physical constraint? but we still can not sport the exact cause.