I guess it could be a validation thing with IPB. I do remember when using IPB 3.0.6 that it checked to see if it was a valid UTF-8 feed. The feed would still work (from what I remember), but it let you know if you was subscribing to a feed that wasn't proper validated to UTF-8. But to be fair, you'll find loads of well known popular feeds that don't validate. The BBC News seem to offer great feeds that are also well validated unlike many other sites out there.
It might also be a case, because your IPB is trying to pull both content and title. That it won't accept a feed without both being offered. No idea, but that could be the case and only IPB themselves will be able to answer that one. You could try finding another site only offering title to be pulled and see if that works also, but not an XenForo site.