vnc4serverI checked the log file at ~/.vnc/machine-cycle:1.log. The log file contained a few error messages about a missing /etc/X11/xserver/SecurityPolicy and missing font directories under /usr/X11R6/lib/X11/fonts/, but I was rather sure that I've seen these messages before, so it wasn't as helpful as I hoped.
I checked the Debian BTS page for vnc4server and my problem was right there as bugs #561619 and #560137. The comments for the latter provide both an explanation (it's related to IPv6 - way over my pretty head), and a workaround that fixed the problem at my end - run the following as root:
sed -i 's/net.ipv6.bindv6only\ =\ 1/net.ipv6.bindv6only\ =\ 0/' /etc/sysctl.d/bindv6only.conf && invoke-rc.d procps restartThat's good enough for me.