Valid Feed gets: Error: Feed has an error or is not valid

Seems like SimplePie is not able to read the feed.

Please post the php code (like <?php RSSImport(10, “https://bueltge.de/feed/&#8221;); ?>) or the shortcode (like [RSSImport display=”5″ feedurl=”https://bueltge.de/feed/”%5D) you use to include your feed!

Hi. The same problem here.

The feed url is: https://www.frd.org.pl/feed/rss
shortcode: [RSSImport feedurl=”https://www.frd.org.pl/feed/rss”%5D
testsite: https://jaworowiludzie.pl/test/

Tested in multiple WordPress installations.
Other feeds work fine.