500 internal server error at GetResponse()

Question

this line to error

 

try
           {
               file = @"" + drives[drive_i] + "fsmanager_good_" + fileto + ".csv";
               string json_off = new WebClient().DownloadString(file);
               byte[] bytes = Encoding.Default.GetBytes(json_off);
               json_off = Encoding.UTF8.GetString(bytes);
               //json_off = Uri.EscapeDataString(json_off);
               json_off = json_off.Replace("Cash & Card", "CashandCard");
               //MessageBox.Show(upurl + "fileupload.php?ontbl=" + Uri.EscapeUriString(json_off));
               string json_on = new WebClient().DownloadString(upurl + "fileupload.php?ontbl=" + Uri.EscapeUriString(json_off));
               if (File.Exists(file))
               {
                   File.Delete(file);
               }
               err = 1;
           }
           catch
           {
              // MessageBox.Show("Please connect internet.", "Online Alert", MessageBoxButtons.OK, MessageBoxIcon.Information);
           }