A nip-05 profile url, like

https://njump.me/dtonon.com

Or a npub one

https://njump.me/npub10000003zmk89narqpczy4ff6rnuht2wu05na7kpnh3mak7z2tqzsv8vwqk

Or a relay

https://njump.me/r/relay.nostr.band

If you feed reader does not auto discover the rss, add ".rss" to the npub or the relay versions:

https://njump.me/npub10000003zmk89narqpczy4ff6rnuht2wu05na7kpnh3mak7z2tqzsv8vwqk.rss

https://njump.me/r/relay.nostr.band.rss 

 tanks, changed the reader, works.
But what is wrong with https://rssviewer.app/ ? 

 I don't know, you could inquiry the service owner.
I suspect a possible problem with the html escaping, it's not fully spec compliant, I will check. 

 That's great! 

 Oooo a relay view, very nice!  I wonder if any clients support opening it..