starting virtual box VM instantly crashes FreeBSD after upgrade from 11 to 12

[Graham Menhennitt]

Hello all,

I recently upgraded my desktop box from 11-Stable to 12-Stable via 
source. I've rebuilt and installed the 5.2.26 virtualbox-ose and 
virtualbox-ose ports from scratch and made sure that no traces of older 
versions exist.

I start the VirtualBox GUI successfully and see the table of my VMs. I 
select one of them and start it and FreeBSD instantly crashes. My screen 
goes blank and eventually FreeBSD reboots.

The only thing I can see in my /var/log/messages is:

   Apr  2 15:07:14 starker kernel: vboxdrv: XXXXXXXXXXXXXXXX VMMR0.r0
   Apr  2 15:07:14 starker kernel: vboxdrv: XXXXXXXXXXXXXXXX VBoxDDR0.r0

after the crash and just before the reboot sequence.

I can't see anything in /usr/ports/UPDATING that's relevant. It was all 
working correctly under 11-Stable.

Does anybody have any clues, please? How can I diagnose further?

Thanks in advance for any help.

Graham