While I recover from the festive season, I just want to notify anyone publishing RSS feeds, especially onto Planet Drupal. You might have implemented your own fix to an RSS recommendation to add certain elements. This still isn't supported by Drupal 7 core, and historically wasn't supported by Views; the fix is quite simple and can be done in your theme layer, so you might have done so.
However! as of Views 7.x-3.12, there has been support for this recommended element by default, pending patching into Drupal 8 core. Unfortunately that might mean that your own fix is adding the element, and its associated XML namespace, twice. My own Planet Drupal feed recently fell foul of this, and needed fixing before it could be reactivated.
So: if you've recently upgraded Views, and if you were smart enough to put such a fix in of your own some time ago, you should now be smart enough to remove it (smarter than me); also, validate your RSS feeds whenever you get a chance.