The 403 error, occurs when you don't grab your chance when you are supposed to start the download (seems over 1 minute). Also occurs when you cancel the save as box and try to download again.

 I've seen this happen: the queue window is refreshed with the main window content: queue over.

 These errors may cause corruptions in your browser session, in other words: trying again won't help. Restart the browser or reboot the entire system and try again.