Work around:
# use the following command to switch to VT1 from outside the VM:
VBoxManage controlvm "Ubuntu 16.04" keyboardputscancode 1D 38 3B 9D B8 CB
# inside the VM, log in and then insert # in front of all variable assignments as to @michael-thayer above:
sudo nano -w /etc/default/console-setup
# finally reboot
sudo reboot
Work around: console- setup
# use the following command to switch to VT1 from outside the VM:
VBoxManage controlvm "Ubuntu 16.04" keyboardputscancode 1D 38 3B 9D B8 CB
# inside the VM, log in and then insert # in front of all variable assignments as to @michael-thayer above:
sudo nano -w /etc/default/
# finally reboot
sudo reboot