The method you are using to access the File is not allowed. The Page you are Requesting exists but you cannot see it because your own system doesn't understand the format the Page is configured for. The Request must be authorized before it can take place. The Request timed out. For some reason the Server took too much time processing your Request.
Net congestion is the most likely reason.
Too many people wanted the same File at the same time. The Page used to be there, but now it's gone. Your Request is missing a Content-Length Header. The Page you Requested has some sort of precondition set up. That means that if something is a certain way, you can have the Page.