How to solve the problem with POST request over SSL protocol?

Question

C++ / C#

Since yesterday, when making a POST request from a C # application to the site, I began to receive an error:

The underlying connection was closed: An unexpected error occurred on a send.

Followed all suggestions from MSDN
https://support.microsoft.com/en-us/kb/915599
But didn't help.
At the same time, GET requests, requests via the HTTP protocol or curl from another site work.
Moreover, judging by the logs, similar POST requests work for some users.
I tried to run it from another computer - it also does not work, and I received a letter from the user about this problem.
Tried changing PHP version, but neither 5.6.19 nor 7.0.4 versions work.
Are there any ideas how to solve?
Thank you.
PS.
I am attaching the code

using System;
using System.Collections.Specialized;
using System.Net;
using System.Security.Authentication;
using Kstudio.Manager;
using Kstudio.Manager.Settings;

namespace PostRequestTest
{
    public static class Program
    {
        static void Main(string[] args)
        {
            UpdateStart("https");
            UpdateStart("http");
            Console.ReadLine();
        }

        private static void UpdateStart(string protocol)
        {
            Console.WriteLine("Sending a POST data through " + protocol.ToUpper());
            WebInfo.Protocol = protocol;
            ServicePointManager.MaxServicePointIdleTime = 5000 * 10;
            ServicePointManager.Expect100Continue = false;
            try
            {
                var result = GetUpdateVersion();
                //result = result.Remove(result.Length/2);
                if (!string.IsNullOrEmpty(result))
                    result = result.Remove(result.Length / 100) + "...\n\n Success!!";
                Console.WriteLine(result);
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error: " + ex.Message);
            }
            Console.WriteLine("====================\n" );
        }

        public static string GetUpdateVersion()
        {
            var req = new ExtWebClient
            {
                Encoding = System.Text.Encoding.UTF8,
                PostParam = new NameValueCollection()
            };
            req.PostParam.Add("data", "somedata");
            req.PostParam.Add("product", 1.ToString());

            //ServicePointManager.ServerCertificateValidationCallback += (sender, certificate, chain, sslPolicyErrors) => true;
            try
            {
                byte[] result = req.DownloadData(new System.Uri(WebInfo.CheckUpdateUrl));
                return DeserializeData(result);
            }
            catch (WebException ex)
            {

                Console.WriteLine("Error: " + ex.Message);
            }
            catch (AuthenticationException ex)
            {
                Console.WriteLine("Error: " + ex.Message);
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error: " + ex.Message);
            }
            return null;
        }

        private static string DeserializeData(byte[] result)
        {
            if (result == null || result.Length == 0)
                return string.Empty;

            return System.Text.Encoding.UTF8.GetString(result, 1, result.Length - 1);
        }

    }
}

using System;
using System.Collections.Specialized;
using System.Net;
using System.Text;

namespace Kstudio.Manager
{
    internal sealed class ExtWebClient : WebClient
    {

        public NameValueCollection PostParam { get; set; }
        protected override WebRequest GetWebRequest(Uri address)
        {
            WebRequest tmprequest = base.GetWebRequest(address);

            HttpWebRequest request = tmprequest as HttpWebRequest;

            //Setting KeepAlive to false
            if (request != null)
                request.KeepAlive = false;

            if (request != null && PostParam != null && PostParam.Count > 0)
            {
                StringBuilder postBuilder = new StringBuilder();
                request.Method = "POST";

                //build the post string
                for (int i = 0; i < PostParam.Count; i++)
                {
                    var key = PostParam.GetKey(i);
                    var value = PostParam.Get(i);
                    if (string.IsNullOrEmpty(key) || string.IsNullOrEmpty(value))
                        continue;
                    postBuilder.AppendFormat("{0}={1}", Uri.EscapeDataString(key),
                        Uri.EscapeDataString(value));
                    if (i < PostParam.Count - 1)
                    {
                        postBuilder.Append("&");
                    }
                }
                byte[] postBytes = Encoding.ASCII.GetBytes(postBuilder.ToString());
                request.ContentLength = postBytes.Length;
                //request.Accept = "application/json";
                //request.UserAgent = "curl/7.37.0";
                request.ContentType = "application/x-www-form-urlencoded";

                var stream = request.GetRequestStream();
                if (stream != null)
                {
                    stream.Write(postBytes, 0, postBytes.Length);
                    stream.Close();
                    stream.Dispose();
                }
            }

            return tmprequest;
        }
  
    }
}

Answer 1

The problem "by itself" disappeared after a couple of days.
Perhaps the hoster corrected something or there was a problem with the provider - this explains why my different computers gave one error and there was only one complaint from the client.
Anyway, thank you all.