Ubuntu 16.04 no longer runs after Virtual Box upgrade

I had big difficulties with VBox 5.1 when I tried it a week or two ago.
I have guests of Linux Mint 17.3 and 18, Xubuntu and Ubuntu 16.04, Debian 8.5, and also WinXP, all of which ran faultlessly in VBox 5.0.# versions in my Xubuntu 14.04 as the host and all the 4.3 and 5.0. versions have given me none, or very few problems ever.

When I noticed 5.1 was available I immediately tried it but it had problems with graphics, USBs, and networking. I did not bother trying to solve this but immediately downgraded back to the 5.0.# version and with that version, the problems went away.

Perhaps worth trying that yourself to see if you have the same success with Vbox-5.0.26, the current 5.0 version.