Hi Graham/Colin -- I did see that page before and ran through the items. No problems pinging or connecting via telnet, and a different server configured the same way (at roughly the same time, give or take a couple days) connects just fine. Thought maybe ufw was getting in the way, so I punched a hole for requests to tarsnap's IP (no effect), and disabled it outright (also no effect).
I'm seeing no obvious permission errors (I'm actually running as root), and I had also tried defining a new cache directory, which also didn't make any difference.