This is on XF 1.5 + xfmg 1.1. It's my staging/beta site.
I don't think it's an addon or style issue as I've disabled all addons + tried default style to no avail. The URL obviously ends in a 404.
Using nginx 1.9 + mariadb 11.1 + php 5.6.
I'm using bog standard well documented nginx locations block for the XF install (double checked here). RSS is not cached either.