X server doesn't start after upgrading vbox lubuntu 17.10 to 18.04

Question

I upgraded a virtualbox from lubuntu 17.10 to 18.04. On reboot, X doesn't start, though I can enter the console via -F1. Attempting to restart doesn't result in anything obviously interesting in /var/log/Xorg.0.log or ~/.xsession-errors, which I have included below.There is console message when I run systemctl restart display-manager, though, `[drm:vbox_bo_unpin [vboxvideo]]

EDIT: I removed the diagnostic output; please see the answer below.

Answer 1

The problem turned out to be that I was running 16.04 on the host, which was getting me virtualbox-5.1. Upgrading to 5.2 on the host fixed my problem.