For example, on Ametys demo on line : http://demo.ametys.org/_plugins/web/www/fr/filter/events/rss.xml, when you see the code source of the page, is missing www in the href of the <atom:link> tag

 <atom:link type="application/rss+xml" rel="self" href="http://demo.ametys.org/_plugins/web/fr/filter/events/rss.xml"/>