RSS feed seems valid, but even W3C can not provide a preview or error. Importing fails

i have a friends RSS-Feed who dont wan't to import the posts into the network (trusted-blogs). We dont have any glue, because w3c validator shows, that it's a valid RSS-Feed, but instead of showing a preview, it shows nothing. So even on w3c is a error somewhere in this rss-feed, but is not shown.

That's the feed: https://die-hirnblutung.de/feed/

W3C Check (normally with preview of the feed): https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fdie-hirnblutung.de%2Ffeed%2F

Maybe its nothing special and easy to fix, but i dont have any idea, as i am not working much with rss-feeds.

https://stackoverflow.com/questions/65839159/rss-feed-seems-valid-but-even-w3c-can-not-provide-a-preview-or-error-importing January 22, 2021 at 12:06PM