Sorry about this.
To manually fix, login into a virtual console.
The VirtualBox default isRight Ctrl + F1
.
Then run the following commands. Delete the offending config file:
sudo rm /etc/default/grub.d/40_only_allow_signed_modules.cfg
Update grub.
sudo update-grub
Reboot.
sudo reboot
Should be fixed after reboot.
Fix through apt-get upgrades coming soon.
(For technical background, see: enforce kernel module software signature verification [module signing] / disallow kernel module loading by default - #23 by Patrick)