Unable to subscribe to the feed

[feutl]

I am unable to subscribe to the feed of my own homepage using a app hosted on my NextCloud installation.
The Demo News Feed just works fine. Also using my own newsfeed with Outlook or Thunderbird works. I am little out of ideas, because even the W3 validator is not giving me a proper hint.

I also started a thread at the Nextcloud News App help forum, but I though it could be helpful getting in touch with you as well.

My feed URL - https://www.feutl.com/blog.rss
The demo feed which works - https://demo.getgrav.org/blog-skeleton/blog.rss

Perhaps you have some hints I can look at.