xorg crashes on lock screen

[SKempf]

When I return from a locked screen, I get a the crash dialog "System program problem detected". The related file in /var/log crash shows this in the header lines:

ProblemType: Crash
Architecture: amd64
Date: Thu Oct 27 10:33:08 2016
DistroRelease: Ubuntu 16.04
ExecutablePath: /usr/lib/xorg/Xorg
ExecutableTimestamp: 1473867446
ProcCmdline: /usr/lib/xorg/Xorg -core :1 -seat seat0 -auth /var/run/lightdm/root/:1 -nolisten tcp vt8 -novtswitch
ProcCwd: /
ProcEnviron:

It appears that the xorg instance on vt8 is crashing while starting the lock process; the crash file in /var/crash appears just after locking the screen. I have attached the xorg log from the display that is crashing (Xorg.1.log). There is a related output in the main display log (Xorg.0.log):

[ 10248.316] (II) AIGLX: Suspending AIGLX clients for VT switch
[ 10248.350] (II) NOUVEAU(G0): NVLeaveVT is called.
[ 10254.968] (II) AIGLX: Resuming AIGLX clients after VT switch
[ 10254.968] (II) intel(0): switch to mode 1920x1080@60.0 on LVDS2 using pipe 0, position (0, 0), rotation normal, reflection none
[ 10255.002] (II) NOUVEAU(G0): NVEnterVT is called.
[ 10255.340] (--) synaptics: SynPS/2 Synaptics TouchPad: touchpad found

The screen locking appears to be working. VT7 is locked with a re-direct to VT8. I can log back in through VT8 and everything is normal, with the exception of the initial crash and subsequent crash dialog pop-up.

Here is some relevant system information:

Lenovo ThinkPad W520
Description:Ubuntu 16.04.1 LTS
Release:16.04
lightdm: Installed: 1.18.3-0ubuntu1
xorg: Installed: 1:7.7+13ubuntu3
xserver-xorg-video-intel: Installed: 2:2.99.917+git20160325-1ubuntu1.1
xserver-xorg-video-nouveau: Installed: 1:1.0.12-1build2

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.