In these cases we usually recommend contacting the publisher to report that their feed isn’t valid. It usually helps to point them to the Feed Validator output for their feed, as many are unaware that they’re producing invalid feeds:
https://www.feedvalidator.org/check.cgi?url=https%3A%2F%2Fwww.loonopzand.nl%2Factueel%2Frss%2F
Having said that, we do hope to handle such cases better in Full-Text RSS, so the next update should improve things. Can’t say when exactly, but hopefully not too far away.