16.04 as virtualbox guest graphical target broken

About 2 weeks ago, Ubuntu 16.04 LTS stopped working in graphical mode.
My system:
Host: latest MacOS on a MacBook Pro from 2015)
Latest version of Virtualbox (5.1.10)
Ubuntu 16.04; guest additions installed

I have a backup of the latest running .vdi-file.

Instead of starting the graphical login screen in the MacOS-window, the windows stays black and the system makes a full stop.
Starting into multiuser.target is fine.

I updated by using "apt-get update; apt-get upgrade"

Any help is appreciated
Thanks

P.S.: Ubuntu 18.04 works fine in the same setting.