HttpRequestException при использовании прокси

Пытаюсь использовать прокси (с ротацией) с HttpClient - но получаю HttpRequestException

System.Net.Http.HttpRequestException: "The SSL connection could not be established, see inner exception."

Чего не хватает ?

        string proxyHost = "IP"; // скрыл
        string proxyPort = "PORT"; // скрыл

        var proxy = new WebProxy
        {
            Address = new Uri($"http://{proxyHost}:{proxyPort}"),
            BypassProxyOnLocal = false,
            UseDefaultCredentials = false
        };

        HttpClientHandler httpClientHandler = new HttpClientHandler()
        {
            AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate | DecompressionMethods.Brotli,
            AllowAutoRedirect = true,
            Proxy = proxy,
        };

        HttpClient httpClient = new HttpClient(httpClientHandler);

        // Некоторые заголовки
        //...
        //...

        HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, "https://адрес"); // скрыл

        request.Content = new FormUrlEncodedContent(new[]
        {
            new KeyValuePair<string, string>("key1", "value1"), // скрыл
            new KeyValuePair<string, string>("key2", "value2"), // скрыл
            new KeyValuePair<string, string>("key3", "value3"), // скрыл
            new KeyValuePair<string, string>("key4", "value4"), // скрыл
        });

        HttpResponseMessage response = await httpClient.SendAsync(request, HttpCompletionOption.ResponseContentRead);

        return await response.Content.ReadAsStringAsync();

System.Net.Http.HttpRequestException
  HResult=0x80131620
  Сообщение = The SSL connection could not be established, see inner exception.
  Источник = System.Net.Http
  Трассировка стека:
   at System.Net.Http.ConnectHelper.<EstablishSslConnectionAsyncCore>d__4.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at System.Runtime.CompilerServices.ConfiguredValueTaskAwaitable`1.ConfiguredValueTaskAwaiter.GetResult()
   at System.Net.Http.HttpConnectionPool.<ConnectAsync>d__52.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at System.Runtime.CompilerServices.ConfiguredValueTaskAwaitable`1.ConfiguredValueTaskAwaiter.GetResult()
   at System.Net.Http.HttpConnectionPool.<CreateHttp11ConnectionAsync>d__53.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at System.Runtime.CompilerServices.ConfiguredValueTaskAwaitable`1.ConfiguredValueTaskAwaiter.GetResult()
   at System.Net.Http.HttpConnectionPool.<GetHttpConnectionAsync>d__45.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at System.Runtime.CompilerServices.ConfiguredValueTaskAwaitable`1.ConfiguredValueTaskAwaiter.GetResult()
   at System.Net.Http.HttpConnectionPool.<SendWithRetryAsync>d__47.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
   at System.Net.Http.RedirectHandler.<SendAsync>d__4.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
   at System.Net.Http.DecompressionHandler.<SendAsync>d__15.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
   at System.Net.Http.HttpClient.<FinishSendAsyncBuffered>d__70.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at PostTestCore.MainWindow.<Button_Click>d__1.MoveNext() in D:\xxx\Users\xxx\source\repos\PostTestCore\PostTestCore\MainWindow.xaml.cs:line 86

  Изначально это исключение было создано в этом стеке вызовов: 
    [Внешний код]

Внутреннее исключение 1:
IOException: Authentication failed because the remote party has closed the transport stream.

UPD: Пробую другие прокси...

C SSL проблем нет, но c прокси всегда получаю 502 ошибку

    HttpClientHandler httpClientHandler = new HttpClientHandler()
    {
        AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate | DecompressionMethods.Brotli,
        AllowAutoRedirect = true,
        Proxy = proxy,
    };

Response

{StatusCode: 502, ReasonPhrase: 'Bad Gateway', Version: 1.0, Content: System.Net.Http.HttpConnectionResponseContent, Headers:{........}}

Без прокси всё ОК

    HttpClientHandler httpClientHandler = new HttpClientHandler()
    {
        AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate | DecompressionMethods.Brotli,
        AllowAutoRedirect = true
    };

Response

{StatusCode: 200, ReasonPhrase: 'OK', Version: 1.1, Content: System.Net.Http.HttpConnectionResponseContent, Headers:{........}}