Autoblog unable to understand Weebly RSS feed?

I am trying to import a RSS feed from Weebly. The URL is but Autoblog gives this error:

A feed could not be found at A feed with an invalid mime type may fall victim to this error, or SimplePie was unable to auto-discover it.. Use force_feed() if you are certain this URL is a real feed.

(I’m not sure what SimplePie is.) The validate link on the dashboard reports the feed valid (although with a number of warnings). Other RSS readers (e.g., FeedGrabber, Newsblur, can all parse the RSS feed. It seems like only Autoblog has problems with it.