Previously I had YARP set to show both on posts, and feeds with no issues.
I disabled YARP to show on posts, so it would only show on RSS
I started getting server time outs on the RSS feed, and feed validation brought up "error on line 1 at column 1: Document is empty"
Disabling YARP fixed this error.
Likewise re-enabling YARP to show on both posts and feeds fixed this.
If mitcho could have a look into this, it would be great.