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FtpWebRequest request = (FtpWebRequest)FtpWebRequest.Create(new Uri("ftp://.../test.txt"));

            request.Method = WebRequestMethods.Ftp.UploadFile;
            request.Credentials = new NetworkCredential(txtUsername.ToString(), txtPassword.ToString()); //Geht nicht
          //request.Credentials = new NetworkCredential("USERNAME", "PASSWORT"); <---- Würde gehen!
            request.UsePassive = true;
            request.UseBinary = true;
            request.KeepAlive = false;

            //Load the file
            FileStream stream = File.OpenRead("test.txt");
            byte[] buffer = new byte[stream.Length];

            stream.Read(buffer, 0, buffer.Length);
            stream.Close();

            //Upload file
            Stream reqStream = request.GetRequestStream();
            reqStream.Write(buffer, 0, buffer.Length);
            reqStream.Close();					

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request.Credentials = new NetworkCredential(txtUsername.Text, txtPassword.Text);