The page you want is not on the server nor has it ever been on the server. The method you are using to access the file is not allowed. 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. It glutted the server. The page use to be there, but now it's gone. Your request is missing a Content-Length header. The page you requested has some sort of pre-condition set up.