$ sudo lightdm --test-mode --debug
[+0.00s] DEBUG: Logging to /var/log/lightdm/lightdm.log
[+0.00s] DEBUG: Starting Light Display Manager 1.26.0, UID=0 PID=25498
[+0.00s] DEBUG: Loading configuration dirs from /usr/share/lightdm/lightdm.conf.d
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-disable-guest.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-disable-log-backup.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-greeter-wrapper.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-guest-wrapper.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/60-lightdm-gtk-greeter.conf
[+0.00s] DEBUG: Loading configuration dirs from /usr/local/share/lightdm/lightdm.conf.d
[+0.00s] DEBUG: Loading configuration dirs from /etc/xdg/lightdm/lightdm.conf.d
[+0.00s] DEBUG: Loading configuration from /etc/lightdm/lightdm.conf
[+0.00s] DEBUG: Using Xephyr for X servers
[+0.00s] DEBUG: Registered seat module local
[+0.00s] DEBUG: Registered seat module xremote
[+0.00s] DEBUG: Registered seat module unity
[+0.01s] DEBUG: Using D-Bus name org.freedesktop.DisplayManager
[+0.05s] DEBUG: Monitoring logind for seats
[+0.05s] DEBUG: New seat added from logind: seat0
[+0.05s] DEBUG: Seat seat0: Loading properties from config section Seat:*
[+0.05s] DEBUG: Seat seat0: Starting
[+0.05s] DEBUG: Seat seat0: Creating user session
[+0.06s] DEBUG: Loading users from org.freedesktop.Accounts
[+0.06s] DEBUG: User /org/freedesktop/Accounts/User1000 added
[+0.09s] DEBUG: Seat seat0: Creating display server of type x
[+0.10s] DEBUG: Using VT 7
[+0.10s] DEBUG: Seat seat0: Starting local X display on VT 7
[+0.10s] DEBUG: XServer 1: Logging to /var/log/lightdm/x-1.log
[+0.10s] DEBUG: XServer 1: Can't launch X server Xephyr, not found in path
[+0.11s] DEBUG: XServer 1: X server stopped
[+0.11s] DEBUG: Releasing VT 7
[+0.11s] DEBUG: Seat seat0: Display server stopped
[+0.11s] DEBUG: Seat seat0: Can't create display server for automatic login
[+0.11s] DEBUG: Seat seat0: Session stopped
[+0.11s] DEBUG: Seat seat0: Stopping display server, no sessions require it
[+0.11s] DEBUG: Seat seat0: Creating greeter session
[+0.11s] DEBUG: Seat seat0: Creating display server of type x
[+0.11s] DEBUG: Using VT 7
[+0.11s] DEBUG: Seat seat0: Starting local X display on VT 7
[+0.11s] DEBUG: XServer 1: Logging to /var/log/lightdm/x-1.log
[+0.11s] DEBUG: XServer 1: Can't launch X server Xephyr, not found in path
[+0.11s] DEBUG: XServer 1: X server stopped
[+0.11s] DEBUG: Releasing VT 7
[+0.11s] DEBUG: Seat seat0: Display server stopped
[+0.11s] DEBUG: Seat seat0: Can't create display server for greeter
[+0.11s] DEBUG: Seat seat0: Session stopped
[+0.11s] DEBUG: Seat seat0: Stopping display server, no sessions require it
[+0.11s] DEBUG: Seat seat0: Stopping
[+0.11s] DEBUG: Seat seat0: Stopped
[+0.11s] DEBUG: Failed to start seat: seat0