c# CloudKit Server 2 Server API

I have tried it a few Days with no success if using the Build-In Functions. Now a tried it with the Bouncy Castle Lib (https://www.bouncycastle.org) an then it worked... here is my code for testing:

using System;
using System.Diagnostics;
using System.IO;
using System.Net;
using System.Security.Cryptography;
using System.Text;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.OpenSsl;
using Org.BouncyCastle.Security;


namespace CloudKitTester
{
  public class CloudKitHelper
  {
  private string _keyID;
  private string _container;
  private string _environment;
  private AsymmetricCipherKeyPair _dsaKeyPair;
  private string _apiVersion = "1";




  public CloudKitHelper(string pemFilePath, string keyID, string container, string environment = "development")
  {
  _keyID = keyID;
  _container = container;
  _environment = environment;


  using (var reader = File.OpenText(pemFilePath))
  {
  var pemReader = new PemReader(reader);
  _dsaKeyPair = pemReader.ReadObject() as AsymmetricCipherKeyPair;
  }
  }


  public string Request(string query, string jsonData = "")
  {
  SHA256Cng sha256 = new SHA256Cng();


  var serverPath = String.Format("/database/{0}/{1}/{2}/public/{3}", _apiVersion, _container, _environment, query);
  var requestBodyBinary = Encoding.UTF8.GetBytes(jsonData);
  var requestBodyHashBase64 = Convert.ToBase64String(sha256.ComputeHash(requestBodyBinary));
  var time = String.Format("{0}Z", DateTime.UtcNow.ToString("s"));


  var signatureString = String.Format("{0}:{1}:{2}", time, requestBodyHashBase64, serverPath);
  var signatureBinary = Encoding.UTF8.GetBytes(signatureString);


  var ecDSASigner = SignerUtilities.GetSigner("SHA256withECDSA");
  ecDSASigner.Init(true, _dsaKeyPair.Private);
  ecDSASigner.BlockUpdate(signatureBinary, 0, signatureBinary.Length);


  var signatureBase64 = Convert.ToBase64String(ecDSASigner.GenerateSignature());


  var request = HttpWebRequest.Create(String.Format("https://api.apple-cloudkit.com{0}", serverPath));
  request.Method = "POST";
  request.ContentType = "text/plain";


  request.Headers.Add("X-Apple-CloudKit-Request-SignatureV1", signatureBase64);
  request.Headers.Add("X-Apple-CloudKit-Request-KeyID", _keyID);
  request.Headers.Add("X-Apple-CloudKit-Request-ISO8601Date", time);


  using (var stream = request.GetRequestStream())
  {
  stream.Write(requestBodyBinary, 0, requestBodyBinary.Length);
  }


  try
  {
  var response = (HttpWebResponse) request.GetResponse();


  using (var output = new StreamReader(response.GetResponseStream()))
  {
  return output.ReadToEnd();
  }
  }
  catch
  {
  return null;
  }
  }
  }

  class Program
  {
  static void Main(string[] args)
  {
  var pathToPemFile = "eckey.pem";
  var keyIDfromCloudKitDashboard = "2c384ea...";
  var cloudKitContainer = "iCloud.com.....";


  var helper = new CloudKitHelper(pathToPemFile, keyIDfromCloudKitDashboard, cloudKitContainer);


  var recordsJSON = helper.Request("records/query", "{\"query\":{\"recordType\":\"News\"}}");


  Debug.Print(recordsJSON);


  }
  }
}