Black screen after 18.04 > 20.04.1 upgrade on virtualbox

After seeing that I got the notification for installing Ubuntu 20.04.1, I thought I’d take a shot at upgrading my work VM (previously Ubuntu 18.04 64 bit on VirtualBox 6.1.14, with Windows 10 as the host OS). After the upgrade, I get the graphical login screen, but then, after logging in, the screen goes black: I cannot use the keyboard or mouse, and I don’t get to see my X / gnome desktop. I could get to my graphical desktop after rebooting and selecting an older kernel from the old Ubuntu: so kernel 4.15 works , while 5.4 doesn’t.

With the focal kernel (non-working case), I can go to a terminal (Ctrl-Alt-F1) and in ps axf I see this:

   1941 ?        SLsl   0:00 /usr/sbin/lightdm
   1974 tty7     Ssl+   0:02  _ /usr/lib/xorg/Xorg -core :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
   2204 ?        Sl     0:00  _ lightdm --session-child 12 19
   2274 ?        Ss     0:00      _ /bin/bash /usr/sbin/lightdm-session /usr/lib/gnome-flashback/gnome-flashback-metacity
   2383 ?        S      0:00          _ xhost +si:localuser:stefan

As far as I can tell, /usr/sbin/lightdm-session runs all scripts in /etc/X11/Xsession.d, and two scripts (probably someone should tell the maintainers about this duplication) try to call xhost for the local user. It seems to me that xhost hangs; I see it basically just writes something to /tmp/.X11-unix/X0 and waits for a reply. Any idea on how to fix this, or debug this further? Shouldn’t Xorg be listening on that socket and replying?