It's an ATOM feed, not just an RSS feed, use an
ATOM Feed Validator (
RSSAtom Alternate) as there can be elements that some rss validators will say are invalid.
Our ATOM feed reads without issue:
https://www.ubbcentral.com/forums/cache/rss1.xmlIt looks like, as Rube pointed out, you're missing your feed language.
Since this is a cached flat file in your cache, after updating your language strings you will have to clear your forum cache to rewrite the feed itself.
Further reading:
Difference between RSS and ATOM