As you say, the new episode appears when you subscribe in iTunes, though the iTunes Store is not showing it yet. I can access the feed OK in FireFox and Terminal; it validates in FeedValidator but CastFeedValidator can't read the feed even after 3 minutes. It's possible the Store may catch up eventually (it may just be slow because the feed hasn't been updated for four years) but there's no way of knowing about that.
All this points to your server failing to respond to some types of request. When I tried in Podcasts Connect it simply said it couldn't read the feed. You got a 504 error - this is a 'GateWay timeout'. This page says:
'A server (not necessarily a Web server) is acting as a gateway or proxy to fulfil the request by the client (e.g. your Web browser) to access the requested URL. This server did not receive a timely response from an upstream server it accessed to deal with your HTTP request.
This usually means that the upstream server is down (no response to the gateway/proxy), rather than that the upstream server and the gateway/proxy do not agree on the protocol for exchanging data.
Fixing 504 errors - general
This problem is entirely due to slow IP communication between back-end computers, possibly including the Web server. Only the people who set up the network at the site which hosts the Web server can fix this problem.'
So your next step is to take it up with the people who are running your server.
