[kirpiit]

Ok, I got it! Or close to that…

I happen to have this patch on my system

https://bbpress.org/forums/topic/updated-to-desmond-but-theme-not-working#post-4254

As I disable that, the magic is done and rss comes back to life!

While I cannot say “what” is wrong with such a patch, I’m happy to confirm that just eliminating it does the trick. In case anybody else has the same issue…

Thanks for the kind help to everybody, with a special thank to kawauso