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.xml
It 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

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