no graphical login prompt and VM lockup after ubuntu upgrade 18.04 to 20.04

I successfully upgraded my Ubuntu 18.04 VM (Virtualbox 6.1 hosted on MacOS) to 20.04, but the gnome display manager login prompt doesn’t appear. I see the ubuntu splash screen, and then it goes black and unresponsive.

X11 worked fine before the upgrade with a default run level of 5.

I can boot with runlevel 3, and manually run startx from tty1 after that without error. If I try telinit 5, then I also see an ubuntu splash screen, then black screen. About a minute after that, any ssh sessions that I have active become unresponsive, and I also cannot tty switch back to the console with F1-6. At that point I have to powerdown the VM, and either let the fsck run or restore from a previous snapshot.

The graphics adapter, as seen in ubuntu is:

   $ lspci -nn | grep -E 'VGA|Display'
   00:02.0 VGA compatible controller [0300]: InnoTek Systemberatung GmbH VirtualBox Graphics Adapter [80ee:beef]

Are there any known issues that would explain this? If not, I’d appreciate some diagnostic tips.