VBox crash when running Linux guests; graphics acceleration not working in Linux Mint

[WhatHaveYou]

I find that sometimes - not always - when I boot my VM I end up with a blank screen. There is a workaround posted by Mint but it does not work. Also, and predictably - actually, the experience just kept getting worse, and has reached the stage of being worse even than I expected - I am getting no joy from Nvidia support.