ERROR: /var/cache/debconf/config.dat is locked by another process: Resource temporarily unavailable
$ sudo fuser /var/cache/debconf/config.dat
/var/cache/debconf/config.dat: 4503
$ ps aux | grep 4503
root 4503 0.0 0.8 10816 9088 ? S 08:39 0:00 /usr/bin/perl -w /usr/share/debconf/frontend /usr/sbin/update-grub
$ sudo kill 4503
After killing the process (4503 in this case) and running update-manager again, the upgrade can be completed successfully.