My instinct would be to check the log files of the server and see what it says happened. I could be wrong, but I can't see how it might be Polymer specific.
I suspect the configuration of the server has changed somehow. Can you get that same file using (for example) 'wget' or 'curl' from the command line? Try to do that while monitoring the server's error and access log files - it depends on your server where these are and what they're called, but on my Ubuntu system, iirc, they're in /var/log/apache2 (I need to use sudo to be able to see their contents, so you might too).
HTH,
Max.