Existe um arquivo chamado .Xauthority no diretório inicial. Alterar o proprietário e o grupo desse arquivo da raiz para o usuário e alterar as permissões para 664 resolveu o problema.
Após inserir as informações de login no lightdm, a tela fica preta por um segundo e retorna para a tela de login novamente. No entanto, consigo fazer login através do TTY e do gdm sem problemas.
Como o arquivo de log em /var/log/lightdm/lightdm.log diz, o problema é "falha de autenticação", mas não consigo descobrir como resolvê-lo.
[+0.01s] DEBUG: Logging to /var/log/lightdm/lightdm.log
[+0.01s] DEBUG: Starting Light Display Manager 1.4.0, UID=0 PID=2279
[+0.01s] DEBUG: Loaded configuration from /etc/lightdm/lightdm.conf
[+0.01s] DEBUG: Using D-Bus name org.freedesktop.DisplayManager
[+0.02s] DEBUG: Registered seat module xlocal
[+0.03s] DEBUG: Registered seat module xremote
[+0.03s] DEBUG: Adding default seat
[+0.03s] DEBUG: Starting seat
[+0.03s] DEBUG: Starting new display for greeter
[+0.03s] DEBUG: Starting local X display
[+0.03s] DEBUG: Using VT 7
[+0.03s] DEBUG: Activating VT 7
[+0.03s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+0.03s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+0.03s] DEBUG: Launching X Server
[+0.03s] DEBUG: Launching process 2287: /usr/bin/X :0 -core -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+0.03s] DEBUG: Waiting for ready signal from X server :0
[+0.03s] DEBUG: Acquired bus name org.freedesktop.DisplayManager
[+0.03s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0
[+0.18s] DEBUG: Got signal 10 from process 2287
[+0.18s] DEBUG: Got signal from X server :0
[+0.18s] DEBUG: Connecting to XServer :0
[+0.18s] DEBUG: Starting greeter
[+0.18s] DEBUG: Started session 2291 with service 'lightdm-greeter', username 'lightdm'
[+0.22s] DEBUG: Session 2291 authentication complete with return value 0: Success
[+0.22s] DEBUG: Greeter authorized
[+0.22s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+0.25s] DEBUG: Session 2291 running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+0.65s] DEBUG: Greeter connected version=1.4.0
[+0.65s] DEBUG: Greeter connected, display is ready
[+0.65s] DEBUG: New display ready, switching to it
[+0.65s] DEBUG: Activating VT 7
[+3.16s] DEBUG: Greeter start authentication for amir
[+3.16s] DEBUG: Started session 2383 with service 'lightdm', username 'amir'
[+3.26s] DEBUG: Greeter start authentication for amir
[+3.26s] DEBUG: Session 2383: Sending SIGTERM
[+3.26s] DEBUG: Started session 2385 with service 'lightdm', username 'amir'
[+3.26s] DEBUG: Session 2383 terminated with signal 15
[+3.26s] DEBUG: Session 2383 failed during authentication
[+3.27s] DEBUG: Session 2385 got 1 message(s) from PAM
[+3.27s] DEBUG: Prompt greeter with 1 message(s)
[+8.48s] DEBUG: Continue authentication
[+8.50s] DEBUG: Session 2385 authentication complete with return value 0: Success
[+8.50s] DEBUG: Authenticate result for user amir: Success
[+8.51s] DEBUG: User amir authorized
[+8.52s] DEBUG: Greeter requests session xubuntu
[+8.52s] DEBUG: Using session xubuntu
[+8.52s] DEBUG: Stopping greeter
[+8.52s] DEBUG: Session 2291: Sending SIGTERM
[+8.76s] DEBUG: Session 2291 exited with return value 0
[+8.76s] DEBUG: Greeter quit
[+8.77s] DEBUG: Dropping privileges to uid 1000
[+8.77s] DEBUG: Calling setresgid
[+8.77s] DEBUG: Calling setresuid
[+8.86s] DEBUG: Restoring privileges
[+8.86s] DEBUG: Calling setresuid
[+8.86s] DEBUG: Calling setresgid
[+8.87s] DEBUG: Dropping privileges to uid 1000
[+8.87s] DEBUG: Calling setresgid
[+8.87s] DEBUG: Calling setresuid
[+8.87s] DEBUG: Writing /home/amir/.dmrc
[+9.07s] DEBUG: Restoring privileges
[+9.07s] DEBUG: Calling setresuid
[+9.07s] DEBUG: Calling setresgid
[+9.28s] DEBUG: Starting session xubuntu as user amir
[+9.28s] DEBUG: Session 2385 running command /usr/sbin/lightdm-session startxfce4
[+9.29s] DEBUG: Registering session with bus path /org/freedesktop/DisplayManager/Session0
[+9.40s] DEBUG: Session 2385 exited with return value 1
[+9.40s] DEBUG: User session quit
[+9.40s] DEBUG: Stopping display
[+9.40s] DEBUG: Sending signal 15 to process 2287
[+10.19s] DEBUG: Process 2287 exited with return value 0
[+10.19s] DEBUG: X server stopped
[+10.19s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+10.19s] DEBUG: Releasing VT 7
[+10.19s] DEBUG: Display server stopped
[+10.19s] DEBUG: Display stopped
[+10.19s] DEBUG: Active display stopped, switching to greeter
[+10.19s] DEBUG: Switching to greeter
[+10.19s] DEBUG: Starting new display for greeter
[+10.19s] DEBUG: Starting local X display
[+10.19s] DEBUG: Using VT 7
[+10.19s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+10.19s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+10.19s] DEBUG: Launching X Server
[+10.19s] DEBUG: Launching process 2494: /usr/bin/X :0 -core -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+10.19s] DEBUG: Waiting for ready signal from X server :0
[+10.34s] DEBUG: Got signal 10 from process 2494
[+10.34s] DEBUG: Got signal from X server :0
[+10.34s] DEBUG: Connecting to XServer :0
[+10.34s] DEBUG: Starting greeter
[+10.34s] DEBUG: Started session 2497 with service 'lightdm-greeter', username 'lightdm'
[+10.36s] DEBUG: Session 2497 authentication complete with return value 0: Success
[+10.36s] DEBUG: Greeter authorized
[+10.36s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+10.36s] DEBUG: Session 2497 running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+10.63s] DEBUG: Greeter connected version=1.4.0
[+10.63s] DEBUG: Greeter connected, display is ready
[+10.63s] DEBUG: New display ready, switching to it
[+10.63s] DEBUG: Activating VT 7
[+10.63s] DEBUG: Stopping greeter display being switched from
[+10.92s] DEBUG: Greeter start authentication for amir
[+10.92s] DEBUG: Started session 2582 with service 'lightdm', username 'amir'
[+10.92s] DEBUG: Session 2582 got 1 message(s) from PAM
[+10.92s] DEBUG: Prompt greeter with 1 message(s)
[+11.20s] DEBUG: Greeter start authentication for amir
[+11.20s] DEBUG: Session 2582: Sending SIGTERM
[+11.20s] DEBUG: Started session 2584 with service 'lightdm', username 'amir'
[+11.20s] DEBUG: Session 2582 terminated with signal 15
[+11.20s] DEBUG: Session 2582 failed during authentication
[+11.21s] DEBUG: Session 2584 got 1 message(s) from PAM
[+11.21s] DEBUG: Prompt greeter with 1 message(s)
[+14.61s] DEBUG: Greeter closed communication channel
[+59.09s] DEBUG: Continue authentication
[+59.11s] DEBUG: Got signal 15 from process 1
[+59.11s] DEBUG: Caught Terminated signal, shutting down
[+59.11s] DEBUG: Stopping display manager
[+59.11s] DEBUG: Stopping seat
[+59.11s] DEBUG: Stopping display
[+59.11s] DEBUG: Session 2497: Sending SIGTERM
[+59.11s] DEBUG: Session 2584 terminated with signal 15
[+59.11s] DEBUG: Session 2584 failed during authentication
[+59.11s] DEBUG: Authenticate result for user amir: Authentication stopped before completion
[+59.23s] DEBUG: Greeter closed communication channel
[+59.23s] DEBUG: Session 2497 exited with return value 0
[+59.23s] DEBUG: Greeter quit
[+59.23s] DEBUG: Sending signal 15 to process 2494
[+59.25s] DEBUG: Process 2494 exited with return value 0
[+59.25s] DEBUG: X server stopped
[+59.25s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+59.25s] DEBUG: Releasing VT 7
[+59.25s] DEBUG: Display server stopped
[+59.25s] DEBUG: Display stopped
[+59.25s] DEBUG: Seat stopped
[+59.25s] DEBUG: Display manager stopped
[+59.25s] DEBUG: Stopping daemon
[+59.25s] DEBUG: Exiting with return value 0
Existe um arquivo chamado .Xauthority no diretório inicial. Alterar o proprietário e o grupo desse arquivo da raiz para o usuário e alterar as permissões para 664 resolveu o problema.