I noticed an RSS feed that had stopped working, and when I checked the validator, it comes up with the handy error:
Error when fetching content over HTTPs: "HTTPs server has incorrect certificate configuration"
This is no good, especially because the actual feed still serves XML just fine. A quick and surprisingly functional solution is to run the link through Google's FeedBurner, a service that somehow hasn't been consigned to their massive graveyard.
It generated a url that looks like "https://feeds.feedburner.com/feedname/11DigitCode", and the RSS Feeder I tested it with seems to work fine.