I found and fixed many problems with this extension, see
https://www.mediawiki.org/wiki/Extension:RSS and
especially
https://www.mediawiki.org/wiki/Special:Code/MediaWiki/112465
Main reasons (since years) were: not respecting xml namespaces and not
respecting rss feed url redirect(s).
A parameter for the number of redirects (zero by default) has been added.