System.Net.WebException in simple HTTPS Request

I have got a problem with C# and a simple HTTPS-Request...

I want to request this URL: https://api.sipgate.com/v1/ In a webbrowser it works fine, but my C#-Code doesn't work :( Has anybody an idea what I did wrong? Thank you!

using System.Net;using System.IO;// [...]WebRequest req = WebRequest.Create("https://api.sipgate.com/v1");req.ContentType = "application/json";try {    WebResponse res = req.GetResponse();  // Exception here...    Stream content = res.GetResponseStream();    Console.WriteLine((new StreamReader(content)).ReadToEnd());} catch (Exception e) {    Console.WriteLine(e.ToString());                }

Here is the exception:

System.Net.WebException: Die zugrunde liegende Verbindung wurde geschlossen: Unerwarteter Fehler beim Senden.. ---> System.IO.IOException: Fehler bei Authentifizierung, da die Gegenseite den Transportstream geschlossen hat.   bei System.Net.Security.SslState.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest)   bei System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)   bei System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest)   bei System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult)   bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)   bei System.Net.TlsStream.ProcessAuthentication(LazyAsyncResult result)   bei System.Net.TlsStream.Write(Byte[] buffer, Int32 offset, Int32 size)   bei System.Net.PooledStream.Write(Byte[] buffer, Int32 offset, Int32 size)   bei System.Net.ConnectStream.WriteHeaders(Boolean async)   --- Ende der internen Ausnahmestapelüberwachung ---   bei System.Net.HttpWebRequest.GetResponse()   bei Sipgate_Test.Program.Main(String[] args) in c:\users\abc\documents\visual studio 2017\Projects\def\hij\Program.cs:Zeile 24.

System.Net.WebException:

Error in authentication because the other side has closed the transport stream.