Sorry, I posted too soon.
Post reboot:
root@HMPBgZ:~# apt-get update
Fetched 51.5 kB in 1s (46.4 kB/s)
Reading package lists... Done
root@HMPBgZ:~# apt-get upgrade
E: dpkg was interrupted, you must manually run 'dpkg --configure -a' to correct
the problem.
root@HMPBgZ:~# dpkg -s libjson-c
This is not fixed with the updated package.
Attempts to install libjson-c2_0.11-4ubuntu2.2_amd64.deb result in the
same lockup which can only be recovered with a hard reboot.
root@HMPBgZ:~# dpkg -i libjson-c2_0.11-4ubuntu2.2_amd64.deb
(Reading database ... 154353 files and directories currently
