Comment 3 for bug 1629701

.xsession-errors contains:

Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Script for ibus started at run_im.
Script for auto started at run_im.
Script for default started at run_im.
init: gnome-session (Unity) main process (2719) terminated with status 1
init: Disconnected from notified D-Bus bus
init: logrotate main process (2579) killed by TERM signal
init: update-notifier-crash (/var/crash/susres.2016-10-03_08:56:40.650760.crash) main process (2613) killed by TERM signal
init: update-notifier-crash (/var/crash/_usr_bin_eog.1001.crash) main process (2615) killed by TERM signal
init: update-notifier-crash (/var/crash/_usr_lib_gvfs_gvfsd-afp.1001.crash) main process (2616) killed by TERM signal
init: update-notifier-crash (/var/crash/_usr_lib_gvfs_gvfsd-afp-browse.1001.crash) main process (2625) killed by TERM signal
init: update-notifier-crash (/var/crash/_usr_lib_ibus_ibus-ui-gtk3.1001.crash) main process (2626) killed by TERM signal
init: update-notifier-crash (/var/crash/_usr_lib_virtualbox_VirtualBox.0.crash) main process (2627) killed by TERM signal
init: update-notifier-crash (/var/crash/_usr_share_software-center_software-center.1001.crash) main process (2628) killed by TERM signal
init: update-notifier-crash (/var/crash/_usr_bin_compiz.1001.crash) main process (2614) killed by TERM signal
init: xsession-init main process (2705) killed by TERM signal
init: hud main process (2712) killed by TERM signal
init: unity-panel-service main process (2732) killed by TERM signal
init: window-stack-bridge main process (2609) killed by KILL signal

Critical line is
"gnome-session (Unity) main process (2719) terminated with status 1"

Similar posts on Ubuntu blogs point to a graphics driver incompatibility - in my case caused by the recent substantial upgrade.