λ /var/log$ sudo tail lightdm/x-0-greeter.log
[+0,00s] DEBUG: unity-greeter.vala:513: Loading command line options
[+0,00s] DEBUG: unity-greeter.vala:541: Setting GTK+ settings
[+0,06s] DEBUG: unity-greeter.vala:564: Creating Unity Greeter
[+0,06s] DEBUG: unity-greeter.vala:57: Creating background surface
[+0,06s] DEBUG: Connecting to display manager...
[+0,06s] DEBUG: Wrote 18 bytes to daemon
[+0,06s] DEBUG: Read 8 bytes from daemon
[+0,06s] DEBUG: Read 150 bytes from daemon
[+0,06s] DEBUG: Connected version=1.10.5 default-session=ubuntu show-manual-login=false hide-users=false has-guest-account=true show-remote-login=true
/usr/sbin/unity-greeter: symbol lookup error: /usr/lib/x86_64-linux-gnu/libgdk-3.so.0: undefined symbol: cairo_surface_set_device_scale