Public bug reported:

I sometimes get the "low-graphics mode" message when starting an Ubuntu
16.04 virtual machine inside of VirtualBox (current development code
trunk, somewhere past version 5.1 beta 3 and a debug build).  When I
switch to VT-7 I find the X server running without any visible issues (I
am the upstream developer of the guest graphics drivers, so I have a
reasonable but not infallible understanding of the X server).

The virtual machine has Guest Additions installed, same vintage and also
a debug build.  Two screens are enabled as is 3D acceleration.  The host
system is also Ubuntu 16.04.  I am still trying to work out what
triggers the message, but in my last attempts a cold boot failed to
produce it, while a clean reboot did, but only when the host system was
not under load (under load as in building VirtualBox).

Searching the lightdm log files led me to the message "CRITICAL **:
session_get_login1_session_id: assertion 'session != NULL' failed",
which in turn led me to this similar-looking Debian bug:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=814760

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: lightdm 1.18.1-0ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-28.47-generic 4.4.13
Uname: Linux 4.4.0-28-generic x86_64
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Jul  7 10:49:27 2016
InstallationDate: Installed on 2016-06-08 (28 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)

** Affects: lightdm (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug xenial

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https://bugs.launchpad.net/bugs/1599775

Title:
  "low-graphics mode" in VirtualBox guest, but X server running

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