WordPress RSS Widget says WordPress RSS Feed is Invalid

For some reason, WordPress is giving an error stating it cannot find a WordPress RSS feed although it exists.

Some of the feeds that don’t work in RSS Widget:

http://relationships.cubeweek.com/feed/
http://debitalliance.com/feed/
http://consumer.alliancepay.com/feed/

Error being received:

RSS Error: A feed could not be found at http://relationships.cubeweek.com/feed/. 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.

Suggestions for making it work?

Any idea why it is not working? How do you use force_feed() to override the error? Thanks.