问无法连接到远程服务器(ftp在开发中工作，但在生产中不起作用)

EN

using System.IO;
using System.Net;
using System.Net.NetworkInformation;
using System.Net.Sockets;

private IEnumerable<SelectListItem> GetFolders()
{
    try
    {
        FtpWebRequest request = (FtpWebRequest)WebRequest.Create(ftpUrl);
        request.Method = WebRequestMethods.Ftp.ListDirectory;
        request.UseBinary = true;
        request.KeepAlive = false;
        request.Timeout = -1;
        request.UsePassive = true;
        request.Proxy = null;
        request.Credentials = new NetworkCredential(ftpUserID, ftpPassword);
        List<SelectListItem> folders = new List<SelectListItem>();
        FtpWebResponse response = (FtpWebResponse)request.GetResponse();
        Stream responseStream = response.GetResponseStream();
        StreamReader reader = new StreamReader(responseStream);
        string names = reader.ReadToEnd();

        reader.Close();
        response.Close();

        var folderList =
            new List<string>(names.Split(new string[] { "\r\n" }, StringSplitOptions.RemoveEmptyEntries)
                .ToList());
        folders = folderList.ConvertAll(a =>
        {
            return new SelectListItem()
            {
                Text = a.ToString(),
                Value = a.ToString(),
                Selected = false
            };
        });
        return folders;
    }
    catch (Exception ex)
    {
        var hostname = (new Uri(ftpUrl)).Host;
        bool pingable = false;
        using (Ping pinger = new Ping())
        {
            try
            {
                PingReply reply = pinger.Send(hostname);
                pingable = reply.Status == IPStatus.Success;
            }
            catch (Exception pEx)
            {
                throw new Exception($"Error pinging {hostname}: {pEx.ToString()}", pEx);
            }
        }

        if (pingable)
        {
            using (TcpClient tc = new TcpClient())
            {
                tc.Connect(hostname, 20);
                bool stat = tc.Connected;
                if (!stat)
                    throw new Exception($"Cannot connect to {hostname} on port 20", ex);

                tc.Close();
            }
            using (TcpClient tc = new TcpClient())
            {
                tc.Connect(hostname, 21);
                bool stat = tc.Connected;
                if (!stat)
                    throw new Exception($"Cannot connect to {hostname} on port 21", ex);

                tc.Close();
            }
            throw new Exception($"Could connect to {hostname} on port 20 and 21", ex);
        }
        throw new Exception($"Could not ping {hostname}", ex);
    }
}