I spent a lot of time fixing this; I hope this helps someone.
The scenario: when Permalinks is off, I can access the RSS Feed url "/?feed=rss2" without error, but enabling the Permalinks the above returns the feed content and after this, an 404 error code. It is possible see the error code response under network browser inspection tools, like "Web Developer" plugin of Chrome.
Well, the feed client does not understand this error code and raises an exception, breaking the consumption page.
I traced the problem to the WP Super Cache. I think the cache does not generate the feed response content, but it tries to get the cache file and it don´t exist, resulting in error 404.
The simple solution, disable feed cache in "Advanced" topic of the "WP Super Cache Settings", checking "Feeds (is_feed)" options and save.
Perhaps your issue is in WP Super Cache.