После включения безопасности в сеансе FTP появляется синтаксическая ошибка 500, команда не распознана

Я использую этот код для подключения моего сервера и загрузки файлов. Работает нормально.

public void downloadFile(object args)
{
    writeStream = null;
    response = null;
    reader = null;
    int dataLength = 0;

    try
    {
        Array argArray = new object[3];
        argArray = (Array)args;
        ProgressBar progressBar1 = (ProgressBar)argArray.GetValue(0);
        Label lbProgress = (Label)argArray.GetValue(1);

        FTPInfo ftpInfo = (FTPInfo)argArray.GetValue(2);
        string ipAddress = ftpInfo.IpAddress;

        string path = ftpInfo.Path;
        string fileName = ftpInfo.FileName;

        path = Regex.Replace(path, "_.", "_e");
        fileName = Regex.Replace(fileName, "_.", "_e");

        string uri = null;
        if (path.Equals(""))
        {
            uri = ipAddress + fileName;
        }
        else
        {
            uri = ipAddress + path + "/" + fileName;
        }
        string[] temp = ipAddress.Split('/');
        string ip = "mchmultimedia.com";
        string userName = ftpInfo.UserName;
        string password = ftpInfo.Password;
        downloadedData = new byte[0];
        ftp = new FTPClass(path);
        ftp.FtpServer = ip;
        ftp.FtpUsername = userName;
        ftp.FtpPassword = password;
        ftp.FtpLogin();
        dataLength = (int)ftp.GetFileSize(fileName);
        Logger.LogDebugMessage("DataLength :" + dataLength.ToString());
        ftp.CloseConnection();
        FtpWebRequest request = FtpWebRequest.Create(uri) as FtpWebRequest;
        request.Method = WebRequestMethods.Ftp.DownloadFile;
        request.Credentials = new NetworkCredential(userName, password);
        request.UsePassive = true;
        request.UseBinary = true;
        request.KeepAlive = false;

        //Set up progress bar
        UpdateProgressBarValue(progressBar1, 0);
        SetProgressBarMaxValue(progressBar1, dataLength);

        response = request.GetResponse() as FtpWebResponse;
        reader = response.GetResponseStream();

        if (!Directory.Exists(GlobalClass.ZIPPED_FOLDER))
            Directory.CreateDirectory(GlobalClass.ZIPPED_FOLDER);
        writeStream = new FileStream(GlobalClass.ZIPPED_FOLDER + "\\" + fileName, FileMode.Create);
        int Length = 2048;
        Byte[] buffer = new Byte[Length];
        int bytesRead = 0;
        Logger.LogDebugMessage("Before while :" + dataLength.ToString());
        while (true)
        {
            Application.DoEvents(); 

            bytesRead = reader.Read(buffer, 0, buffer.Length);
            if (bytesRead == 0)
            {
                try
                {
                    try
                    {
                        reader.Close();
                    }
                    catch (Exception ex)
                    {
                        Logger.LogErrorMessage("reader close if bytesRead ==00", ex);

                    }

                    try
                    {
                        response.Close();
                    }
                    catch (Exception ex)
                    {
                        Logger.LogErrorMessage("response close if  bytesRead ==00", ex);
                    }

                    try
                    {
                        writeStream.Close();
                    }
                    catch (Exception ex)
                    {
                        Logger.LogErrorMessage("writeStream close if  bytesRead ==00", ex);
                    }

                }
                catch (Exception ex)
                {

                }
                UpdateProgressBarValue(progressBar1, progressBar1.Maximum);
                Application.DoEvents();
                break;
            }
            else
            {
                writeStream.Write(buffer, 0, bytesRead);

                if (progressBar1.Value + bytesRead <= progressBar1.Maximum)
                {
                    totalBytesRead = progressBar1.Value + bytesRead;
                    int percentage = (int)((double)totalBytesRead / dataLength * 100);
                    UpdateProgressBarValue(progressBar1, totalBytesRead);

                    SetText(lbProgress, "File download " + percentage.ToString() + " % completed");
                    if (percentage == 100)
                    {
                        if (totalBytesRead == dataLength)
                        {
                            try
                            {
                                try
                                {
                                    reader.Close();
                                }
                                catch (Exception ex)
                                {
                                    Logger.LogErrorMessage("reader close if percentage==100", ex);
                                }

                                try
                                {
                                    response.Close();
                                }
                                catch (Exception ex)
                                {
                                    Logger.LogErrorMessage("response close if percentage==100", ex);
                                }

                                try
                                {
                                    writeStream.Close();
                                }
                                catch (Exception ex)
                                {
                                    Logger.LogErrorMessage("writeStream close if percentage==100", ex);
                                }

                            }
                            catch (Exception ex)
                            {

                            }
                            SetText(lbProgress, "File download successfully completed,Please wait...");
                            unzipFile(fileName);
                        }

                    }

                    RefreshProgressBar(progressBar1);
                    Application.DoEvents();
                }

            }
        }
    }
    catch (System.Threading.ThreadAbortException ex)
    {
        if (thread != null)
        {
            thread.Abort();
        }
        Logger.LogErrorMessage("ThreadAbortException", ex);

    }
    catch (Exception ex)
    {
        Logger.LogErrorMessage("Exception there was an error connecting", ex);
        Logger.ReportBug("There was an error connecting to the FTP Server.", ex);
    }
    finally
    {
        try
        {
            try
            {
                reader.Close();
            }
            catch (Exception ex)
            {
                Logger.LogErrorMessage("read finally", ex);
            }

            try
            {
                response.Close();
            }
            catch (Exception ex)
            {
                Logger.LogErrorMessage("resonse finally", ex);
            }

            try
            {
                writeStream.Close();
            }
            catch (Exception ex)
            {
                Logger.LogErrorMessage("writeStream finally", ex);
            }
        }
        catch (Exception ex)
        {

        }
    }
}

Но клиенту нужно, чтобы это был безопасный FTP. Итак, я попробовал, установив

request.EnableSsl = true;

как указано в Различия между SFTP и FTP через SSH

И выкидывает:

Удаленный сервер возвратил ошибку: (500) Синтаксическая ошибка, команда не распознана.