It should be working right, as long as the RSS client sends an Accept header equal to application/rss+xml.

(4/4) Remembered I could ask some AI to make things easier.

For kbin.earth feed, Grok says it's Cloudflare bot protection, likely because neither Newsboat or Feeder pass a valid browser agent.

For TBB, Grok for some reason had a hiccup, so asked GPT-5.4 on DuckDuckGo's AI bridge, and it said:

The problem is that the URL is not returning a valid RSS/Atom feed. It’s returning an HTML login page instead, so feed readers see a normal webpage with , , etc., not feed XML. That’s why one reader says “no feeds on the page,” another says “unsupported feed format,” and another says “malformed xml string.”

From the pasted page, the site is protected by login and the feed endpoint appears to require authentication or is redirecting to the login page. The presence of a CSRF token and sign-in form confirms the response is HTML, not feed data.

You can send albums, but unfortunately Mbin can not read them.