For activities at the site level (course id=1), all RSS feeds are currently broken. When enabling the feeds, e.g., in a forum, the RSS icons are shown, but the feeds themselves only return an "RSS error".
The reason is located in rss/file.php, around line 93:
This test does not work as expected for the site-level course, apparently because the "hidden" attribute is unavailable there.
Improved version of the code: