From my notes:
"Whenever you change Host Kernel, do:
sudo apt-get install linux-headers-$(uname -r)
sudo apt-get remove virtualbox-dkms
sudo apt-get install virtualbox-dkms
This forces Vbox kernel modules to re-compile, apparently?
I have yet to change the kernel on my main PC with VBox.
But googling shows this to be a common suggestion after kernel updates.