That was pretty bizarre. I fixed a strange bug in the sidebar logic that was showing multiple other posts in the right column on archived post pages—for some reason the widget was feeding other posts into the “Interesting” feed that blew up the pages. I had to switch to a different plugin last year  (the old one was unsupported and woefully out of date) and I didn’t notice the single post pages were broken until a few weeks ago. Futzing with the logic, that widget is now suppressed on all pages except for the homepage.