Comment 3 for bug 1759452

I've experienced the same issue with the same combination of host/guest. I've tried multiple versions of Virtualbox for Windows. Blank screen after after starting X after taking the latest Ubuntu 16.04 updates on Tuesday, 3/28/2018.

This update replaced virtualbox-guest* 5.0.40 with 5.1.34-dfsg-0ubuntu1.16.04.2:
Unpacking virtualbox-guest-x11 (5.1.34-dfsg-0ubuntu1.16.04.2) over (5.0.40-dfsg-0ubuntu1.16.04.2) ...

host: Windows 10 Enterprise
      Virtualbox 5.1.28, 5.1.34, and 5.2.8 (tried each version)

guest: Kubuntu 16.04
       virtualbox-guest-x11 5.1.34-dfsg-0ubuntu1.16.04.2
       sddm 0.13.0-1ubuntu5

I've already reproduced the same blank screen behavior with a fresh Kubuntu 16.04 install into a fresh virtual machine.

I've found the following things produce improved behavior:

1) uninstalling virtualbox-guest-x11. Not desirable from a functionality perspective, but it does produce working video for both the X display manager and KDE.

2) using xdm instead of sddm as /etc/X11/default-display-manager. Now, with the guest-x11 pkg installed, there's working video in the login screen from xdm, although kde plasma still produces a blank screen.

3) uninstalling virtualbox-guest-x11, installing VBoxLinuxAdditions.run 5.2.8 included in the Virtualbox 5.2.8 installation. Works great with the three vbox modules installed.

4) exporting/importing to VMware Player.

So it would appear it's an issue with the virtualbox-guest-x11 5.1.34-dfsg-0ubuntu1.16.04.2 package that was made available recently in the Ubuntu 16.04 updates repo.

Regards,
Dan