Couldn’t be bothered to back this up by a year to hide it.
Virtual Box on Windows Host, running a Linux Mint 20 Client.

When rebooting the Linux Mint client, this always happens. It only happens when you “RESTART”. Shutting down with “SHUTDOWN” works correctly, oddly enough.

Also this alert always appears, which if of course nonsense since “there are no proprietary drivers in use”.

And here is the video config for the client:

The Guest Additions DVD is not removed when Rclick / Eject is done in the client. So apparently the EJECT message is ignored by the VBox Host software, and on next boot the Guest Additions is still there:

I have removed the Guest Additions DVD by choosing “Remove Media” or some such from the Host menu at the bottom right of the screen. This works as intended. So what’s up with EJECT from inside the client not removing the disk? Supposed to, correct?
Update: Unchecking “enable 3D” in the display settings of the VM fixed the crash on restart. It was always ok to just shut down and then restart from the host VM interface. No crash during a “Shutdown” only a “Restart” oddly. Don’t really need 3D support anyway I guess. Oh, BTW, having 4 cores assigned to the VM client works much better than just one. It was a little laggy with just one core.
Update: After the latest Nvidia driver had been installed on the Windows host, no reboot had been done. I thought perhaps that this had caused the problem. So I rebooted the Windows 10 2004 host and tried the test again: Enable 3D in the guest config, start guest, and then “RESTART” the guest. And it crashed the same way. So that’s not the problem here.
:ww