The update I installed this morning made it impossible for me to launch any VirtualBox VM.
It's remotely possible that it's a local issue, because I may have rebooted after the update while VBox was still running, but that's never affected it before. It's always saved the current VM before allowing the reboot to happen. Besides, it affects ALL of my VMs, so I suspect the fault lies with the VBox manager, not any of the VMs.