I don’t know if this weird trick will work for you (not knowing what’s causing it) but for me it does:
At the GRUB screen select Advanced options, then “(fast but insecure mode)”. Don’t worry it’s not insecure.
That should make the system boot and sddm screen to appear.
Login to desktop and after a short while logout and reboot.
This time select the default 1st line as usual.
Next boots should be fine as well.
No clue why it happens (issue in VirtualBox only btw) nor why the trick fixes it 
Nvidia graphic card in my host too, but not nvidia driver.
Hope it helps.