@Beta-Pictoris tout les paquets lightdm sont bien installés.
Merci pour le fichier de log ! le voici :
[+0.00s] DEBUG: Logging to /var/log/lightdm/lightdm.log
[+0.00s] DEBUG: Starting Light Display Manager 1.28.0, UID=0 PID=7902
[+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-backup-logs.conf
[+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-minimum-vt.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-session-wrapper.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-user-authority-in-system-dir.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: Registered seat module local
[+0.00s] DEBUG: Registered seat module xremote
[+0.00s] DEBUG: Registered seat module unity
[+0.00s] DEBUG: Using D-Bus name org.freedesktop.DisplayManager
[+0.00s] DEBUG: Monitoring logind for seats
[+0.00s] DEBUG: New seat added from logind: seat0
[+0.00s] DEBUG: Seat seat0: Loading properties from config section Seat:*
[+0.00s] DEBUG: Seat seat0: Starting
[+0.00s] DEBUG: Seat seat0: Creating greeter session
[+0.00s] DEBUG: Seat seat0: Creating display server of type x
[+0.01s] DEBUG: posix_spawn avoided (fd close requested)
[+0.01s] DEBUG: posix_spawn avoided (fd close requested)
[+0.01s] DEBUG: Quitting Plymouth
[+0.01s] DEBUG: posix_spawn avoided (fd close requested)
[+0.02s] DEBUG: Using VT 1
[+0.02s] DEBUG: Seat seat0: Starting local X display on VT 1
[+0.02s] DEBUG: XServer 0: Logging to /var/log/lightdm/x-0.log
[+0.02s] DEBUG: XServer 0: Writing X server authority to /var/run/lightdm/root/:0
[+0.02s] DEBUG: XServer 0: Launching X Server
[+0.02s] DEBUG: Launching process 7914: /usr/bin/X -core -noreset :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt1 -novtswitch
[+0.02s] DEBUG: XServer 0: Waiting for ready signal from X server :0
[+0.02s] DEBUG: Acquired bus name org.freedesktop.DisplayManager
[+0.02s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0
[+0.14s] DEBUG: Loading users from org.freedesktop.Accounts
[+0.14s] DEBUG: User /org/freedesktop/Accounts/User1000 added
[+0.16s] DEBUG: posix_spawn avoided (automatic reaping requested) (fd close requested)
[+0.29s] DEBUG: Seat seat0 changes active session to
[+1.11s] DEBUG: Got signal 10 from process 7914
[+1.11s] DEBUG: XServer 0: Got signal from X server :0
[+1.11s] DEBUG: XServer 0: Connecting to XServer :0
[+1.11s] DEBUG: posix_spawn avoided (fd close requested) (child_setup specified)
[+1.11s] DEBUG: Seat seat0: Display server ready, starting session authentication
[+1.11s] DEBUG: Session pid=7997: Started with service 'lightdm-greeter', username 'lightdm'
[+1.13s] DEBUG: Session pid=7997: Authentication complete with return value 0: Success
[+1.13s] DEBUG: Seat seat0: Session authenticated, running command
[+1.13s] DEBUG: Session pid=7997: Running command /usr/sbin/lightdm-gtk-greeter
[+1.13s] DEBUG: Creating shared data directory /var/lib/lightdm-data/lightdm
[+1.13s] DEBUG: Session pid=7997: Logging to /var/log/lightdm/seat0-greeter.log
[+1.32s] DEBUG: Activating VT 1
[+1.32s] DEBUG: Activating login1 session c2
[+1.33s] DEBUG: Seat seat0 changes active session to c2
[+1.33s] DEBUG: Session c2 is already active
[+1.51s] DEBUG: Greeter connected version=1.28.0 api=1 resettable=false
[+2.05s] DEBUG: Greeter start authentication for tbarnouin
[+2.05s] DEBUG: Session pid=8144: Started with service 'lightdm', username 'tbarnouin'
[+2.16s] DEBUG: Session pid=8144: Got 1 message(s) from PAM
[+2.16s] DEBUG: Prompt greeter with 1 message(s)
[+3.68s] DEBUG: Seat seat0 changes active session to 2
[+16.52s] DEBUG: Seat seat0 changes active session to 4
[+17.38s] DEBUG: Seat seat0 changes active session to 2
[+18.00s] DEBUG: Seat seat0 changes active session to c2
[+18.00s] DEBUG: Session c2 is already active
[+24.06s] DEBUG: Seat seat0 changes active session to 2
[+307.25s] DEBUG: Seat seat0 changes active session to 4
[+308.31s] DEBUG: Seat seat0 changes active session to 2
[+310.47s] DEBUG: Seat seat0 changes active session to c2
[+310.47s] DEBUG: Session c2 is already active
[+316.84s] DEBUG: Seat seat0 changes active session to 2
[+319.76s] DEBUG: Seat seat0 changes active session to
[+321.22s] DEBUG: Seat seat0 changes active session to 4
[+327.63s] DEBUG: Seat seat0 changes active session to 2
[+2377.15s] DEBUG: Got signal 15 from process 1
[+2377.15s] DEBUG: Caught Terminated signal, shutting down
[+2377.15s] DEBUG: Stopping display manager
[+2377.15s] DEBUG: Seat seat0: Stopping
[+2377.15s] DEBUG: Seat seat0: Stopping display server
[+2377.15s] DEBUG: Sending signal 15 to process 7914
[+2377.15s] DEBUG: Seat seat0: Stopping session
[+2377.15s] DEBUG: Terminating login1 session c2
[+2377.30s] DEBUG: Session pid=7997: Sending SIGTERM
[+2377.30s] DEBUG: Seat seat0: Stopping session
[+2377.30s] DEBUG: Session pid=8144: Sending SIGTERM
[+2377.30s] DEBUG: Session pid=8144: Terminated with signal 15
[+2377.30s] DEBUG: Session: Failed during authentication
[+2377.30s] DEBUG: Seat seat0: Session stopped
[+2377.30s] DEBUG: Greeter closed communication channel
[+2377.30s] DEBUG: Session pid=7997: Exited with return value 0
[+2377.30s] DEBUG: Seat seat0: Session stopped
[+2377.32s] DEBUG: Process 7914 terminated with signal 6
[+2377.32s] DEBUG: XServer 0: X server stopped
[+2377.32s] DEBUG: Releasing VT 1
[+2377.32s] DEBUG: XServer 0: Removing X server authority /var/run/lightdm/root/:0
[+2377.32s] DEBUG: Seat seat0: Display server stopped
[+2377.32s] DEBUG: Seat seat0: Stopped
[+2377.32s] DEBUG: Display manager stopped
[+2377.32s] DEBUG: Stopping daemon
[+2377.32s] DEBUG: Exiting with return value 0