Umm... there isn't an RSS link on the default main page... so where are you getting this link from? It's not a valid jForum link so the 404 is correct.
A quick look at the RSSAction codes shows that there are only 3 types of RSS feeds: RecentTopics, Forum Topics, and Topic Posts. There is no code for a "main page" RSS feed. [originally posted on jforum.net by monroe]