I have the same Issue. Adding the line Response.Close() works for me.

Response.BufferOutput = false;
  Response.Clear();
  Response.ContentType = "text/csv";
  Response.AppendHeader("Content-Disposition", "attachment; filename=test.csv");
  Response.TransmitFile(PathFile);
  Response.Flush();
  Response.SuppressContent = false;
  Response.Close();
  HttpContext.Current.ApplicationInstance.CompleteRequest();