Q: My feed isn't updating: "Can’t read your feed. null"

I'm having trouble refreshing my podcast feed in iTunes, http://ssir.org/podcasts/rss

It was up and working for several years.  Then I took a long break.  When i started podcasting again, my podcast, still at the same URL, had been removed from the iTunes store.

Now I'm getting this error: "Can’t read your feed. null" and the status is "Error"

I've tried changing the URL slightly, that didn't help.  The RSS feed validates correctly: http://castfeedvalidator.com/?url=http://ssir.org/podcasts/rss

Any suggestions?