多个线程向filezilla上传文件，ftp server返回错误550文件不可用

Problem:当我ftp一次只上传一个文件时，文件上传得很好，但是当我使用多个后台工作人员上传文件到ftp服务器时，我得到异常:

• ex{"远程服务器返回错误:(550)文件不可用(例如，未找到文件，无法访问)。"}系统。异常{System.Net.WebException}

只有一些文件被上传。我很确定文件在那个位置退出，事实上，在另一次运行中，它抱怨的文件不存在被下载，但错误转移到另一个文件。

Code Description:在下面的代码我从一个ftp服务器下载一个文件，并把它放在另一个。这段代码位于BackgroundsWorker_DoWork方法中。后台工作线程是在循环中创建的。

void imageDownloadWorker_DoWork(object sender, DoWorkEventArgs e)
        {
           string[] ftpInfo = (string[])e.Argument;
            try
            {
                ///////////////////////////Downloading///////////////////////////////////////
                string uri = String.Format("ftp://{0}/{1}/images/{2}", ftpInfo[1], ftpInfo[2], ftpInfo[5]);
            FtpWebRequest request = (FtpWebRequest)WebRequest.Create(uri);
            request.Method = WebRequestMethods.Ftp.DownloadFile;
            request.UseBinary = true;
            request.Credentials = new NetworkCredential(ftpInfo[3], ftpInfo[4]);
            FtpWebResponse response = (FtpWebResponse)request.GetResponse();
            Stream ftpStream = response.GetResponseStream();
            long cl = response.ContentLength;
            int bufferSize = 4096;
            int readCount = 0;
            byte[] buffer = new byte[bufferSize];
            MemoryStream memStream = new MemoryStream();
            readCount = ftpStream.Read(buffer, 0, bufferSize);
            while (readCount > 0)
            {
                memStream.Write(buffer, 0, readCount);
                readCount = ftpStream.Read(buffer, 0, bufferSize);
            }
            response.Close();
            ///////////////////////////Uploading///////////////////////////////////////
            string uri1 = String.Format("ftp://{0}/{1}/{2}", "127.0.0.1", string.Empty, ftpInfo[5]);
            FtpWebRequest request1 = (FtpWebRequest)WebRequest.Create(uri1);
            request1.Credentials = new NetworkCredential("user", "password");
            request1.KeepAlive = false;
            request1.Method = WebRequestMethods.Ftp.UploadFile;
            request1.UseBinary = true;
            request1.ContentLength = memStream.Length;
            int buffLength = 4096;
            byte[] buff = new byte[buffLength];
            int contentLen;
            // Stream to which the file to be upload is written
            Stream strm = request1.GetRequestStream();
            memStream.Seek(0, SeekOrigin.Begin);
            contentLen = memStream.Read(buff, 0, buffLength);
            // Till Stream content ends
            while (contentLen != 0)
            {
                // Write Content from the file stream to the FTP Upload Stream
                strm.Write(buff, 0, contentLen);
                contentLen = memStream.Read(buff, 0, buffLength);
            }
            // Close the file stream and the Request Stream
            strm.Close();
            ftpStream.Close();
            memStream.Close();
        }
        catch(Exception ex)
        {
            MessageBox.Show("While Downloading File " + ftpInfo[5] + " " + ex.ToString(), "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            e.Result = null;
            return;
        }

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Edit:在File Zilla Server中有一个选项:常规设置>性能设置>线程数。我把它设置为20，没有任何区别。