It turns out there was a nasty conflict between two WordPress plugins: DISQUS (the commenting system) and Yet Another Related Posts Plugin (which generates suggested articles at the bottom of every post).
Since there are other ways to generated suggested posts, I’ve dropped that plugin for now. DISQUS is more irreplaceable, since it’s the platform that manages the community for this blog.
In fact, it was the wonderful Gianni of DISQUS support that helped me figure out what was wrong after my troubleshooting lead me in her direction.
If you haven’t signed up for DISQUS, you should.