HRESULT 0x80072F7D on a https request with client certificate over Windows.Web.HttpClient from Windows Phone 8.1

I am trying to access a json from a server over Windows.Web.HttpClient from a Windows Phone 8.1 App.

But for security reasons there is a proxy server with client authentication certificates. I am able to set the Client Authentication Certificate but it always fails with an error. The error-message is "Exception from HRESULT: 0x80072F7D"

   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Framework.Mobile.Widgets.LoginPage.<Btn_Login_Click>d__0.MoveNext()

The SSL-Certificate on the server is an wildcard certificate for my entire domain, the client certificate is a self-signed certificate.

Access the Service over the Internet Explorer on Windows Phone works fine, so i think it is a error of the HttpClient. Any Ideas?

mfg harmoniemand