This morning, on two desktop systems with Nvidia graphics, d-u aborted, and the video signal was killed, at the point where the dialog for updating or keeping /etc/systemd/logind.conf is presented. The screen goes black with no signal, and no further hdd activity is seen.
Fix -- wait 5 minutes to make sure all processing is completed, then either press reset or use Alt-PrtSc-RSEIUB if you have it set. On the next grub menu, press "e" to edit, add "3" to the end of your kernel boot line options, and then press Enter Ctrl-x to boot to tty1. Log in as root to tty1, and enter
dpkg --configure -a
and answer the configuration question, and d-u should complete with no further issues. Worked on two systems for me -- YMMV.