HttpWebRequest Failure

I'm trying to consume a WebService via WebRequest in C # and simply can not get any response from the server. However, via Postman I get the answer using the same parameters.

code:

            var JsonCliente = JsonConvert.SerializeObject(clienteMP);
            var payload = UTF8Encoding.UTF8.GetBytes(JsonCliente);

            String username = APIKey;
            String password = "";
            String encoded = System.Convert.ToBase64String(System.Text.Encoding.GetEncoding("ISO-8859-1").GetBytes(username + ":" + password));

            string URLRequest = URI;
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(URLRequest);
            request.Headers.Add("Authorization", "Basic " + encoded);
            request.Method = "POST";
            request.ContentType = "application/json";
            request.Accept = "application/json";

            using (var writer = new StreamWriter(request.GetRequestStream()))
            {
                var settings = new JsonSerializerSettings();

                settings.NullValueHandling = NullValueHandling.Ignore;

                var data = JsonConvert.SerializeObject(clienteMP, Newtonsoft.Json.Formatting.None, settings);
                writer.Write(data);
                writer.Flush();
                writer.Close();
            }

            WebResponse response = request.GetResponse();
            using (Stream stream = response.GetResponseStream())
            {
                StreamReader reader = new StreamReader(stream, Encoding.UTF8);
                dynamic myObject = JsonConvert.DeserializeObject<dynamic>(reader.ReadToEnd());
                return myObject.ToString();
            }

the exception is given in line WebResponse response = request.GetResponse(); , the error message is:

InnerException = {"Termination of existing connection by remote host was forced"}

Message="Unable to read transport connection data: An existing connection was forced by the remote host."