Eu li todas as soluções propostas neste fórum e em outros lugares. Nenhum deles funcionou. Aqui está como os erros de .xsession se parecem:
openConnection: connect: No such file or directory
cannot connect to brltty at :0
upstart: gpg-agent pre-start process (2326) terminated with status 1
upstart: gnome-session (Unity) main process (2413) terminated with status 1
upstart: unity-settings-daemon main process (2398) killed by TERM signal
upstart: logrotate main process (2306) killed by TERM signal
upstart: update-notifier-crash (/var/crash/_usr_lib_x86_64-linux-gnu_indicator-application_indicator-upstart: application-service.1000.crash) main process (2354) killed by TERM signal
upstart: bamfdaemon main process (2399) killed by TERM signal
upstart: unity7 main process (2411) killed by TERM signal
upstart: hud main process (2423) killed by TERM signal
upstart: Disconnected from notified D-Bus bus
upstart: unity-panel-service main process (2421) killed by TERM signal
upstart: window-stack-bridge main process (2332) killed by KILL signal