most likely a network issue or firewall issue. The web server is trying to make a server side web request to get the feed but it is failing to get a response. could be a network problem or a change in firewall settings could be blocking the web request. even though it is on the same server it still is like the server is making a request over the internet.
first step is to check the feed manually with a web browser making the request for the feed url, that is probably working fine but the web server is not able to make a similar request for some reason.