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Lightdm refuses to allow me to login on a new Xubuntu 18.04 installation. After I enter my user name and passwd, instead of logging in, the screen blanks and the greeter re-appears. I'm baffled by this login loop; I don't see anything wrong. Password and user are good. Permissions are correct; ssh works, as does a console login, but the greeter does not. During each loop, the following lines appear in /var/log/syslog:

Jul  8 12:12:42 guy systemd[1]: session-c154.scope: Killing process 6145 (lightdm) with signal SIGTERM.
Jul  8 12:12:42 guy systemd[1]: session-c154.scope: Killing process 6183 (lightdm-greeter) with signal SIGTERM.
Jul  8 12:12:42 guy systemd[1]: session-c154.scope: Killing process 6184 (lightdm-gtk-gre) with signal SIGTERM.
Jul  8 12:12:42 guy systemd[1]: Stopping Session c154 of user lightdm.
Jul  8 12:12:42 guy systemd[1]: Stopped Session c154 of user lightdm.
Jul  8 12:12:42 guy systemd[1]: Created slice User Slice of rack.
Jul  8 12:12:42 guy systemd[1]: Starting User Manager for UID 1011...
Jul  8 12:12:42 guy systemd[1]: Started Session c155 of user rack.
Jul  8 12:12:42 guy systemd[1]: Stopping User Manager for UID 331...
Jul  8 12:12:42 guy systemd[6166]: Stopping Accessibility services bus...
Jul  8 12:12:42 guy systemd[6166]: Stopped target Default.
Jul  8 12:12:42 guy systemd[6166]: Stopping D-Bus User Message Bus...
Jul  8 12:12:42 guy systemd[6166]: Stopping Virtual filesystem service...
Jul  8 12:12:42 guy systemd[6166]: Stopped D-Bus User Message Bus.
Jul  8 12:12:42 guy systemd[6166]: Stopped Accessibility services bus.
Jul  8 12:12:42 guy systemd[6166]: Stopped Virtual filesystem service.
Jul  8 12:12:42 guy systemd[6166]: Stopped target Basic System.
Jul  8 12:12:42 guy systemd[6166]: Stopped target Timers.
Jul  8 12:12:42 guy systemd[6166]: Stopped target Sockets.
Jul  8 12:12:42 guy systemd[6166]: Closed GnuPG cryptographic agent and passphrase cache.
Jul  8 12:12:42 guy systemd[6166]: Closed D-Bus User Message Bus Socket.
Jul  8 12:12:42 guy systemd[6166]: Closed GnuPG cryptographic agent and passphrase cache (access for web browsers).
Jul  8 12:12:42 guy systemd[6166]: Closed GnuPG network certificate management daemon.
Jul  8 12:12:42 guy systemd[6166]: Closed GnuPG cryptographic agent (ssh-agent emulation).
Jul  8 12:12:42 guy systemd[6166]: Closed GnuPG cryptographic agent and passphrase cache (restricted).
Jul  8 12:12:42 guy systemd[6166]: Stopped target Paths.
Jul  8 12:12:42 guy systemd[6166]: Reached target Shutdown.
Jul  8 12:12:42 guy systemd[6166]: Starting Exit the Session...
Jul  8 12:12:42 guy systemd[6166]: Received SIGRTMIN+24 from PID 23237 (kill).
Jul  8 12:12:42 guy systemd[1]: Stopped User Manager for UID 331.
Jul  8 12:12:42 guy systemd[1]: Removed slice User Slice of lightdm.
Jul  8 12:12:42 guy systemd[23224]: Listening on GnuPG cryptographic agent and passphrase cache (access for web browsers).
Jul  8 12:12:42 guy systemd[23224]: Reached target Paths.
Jul  8 12:12:42 guy systemd[23224]: Listening on GnuPG cryptographic agent (ssh-agent emulation).
Jul  8 12:12:42 guy systemd[23224]: Listening on GnuPG network certificate management daemon.
Jul  8 12:12:42 guy systemd[23224]: Listening on GnuPG cryptographic agent and passphrase cache (restricted).
Jul  8 12:12:42 guy systemd[23224]: Reached target Timers.
Jul  8 12:12:42 guy systemd[23224]: Starting D-Bus User Message Bus Socket.
Jul  8 12:12:42 guy systemd[23224]: Listening on GnuPG cryptographic agent and passphrase cache.
Jul  8 12:12:42 guy systemd[23224]: Listening on D-Bus User Message Bus Socket.
Jul  8 12:12:42 guy systemd[23224]: Reached target Sockets.
Jul  8 12:12:42 guy systemd[23224]: Reached target Basic System.
Jul  8 12:12:42 guy systemd[23224]: Reached target Default.
Jul  8 12:12:42 guy systemd[23224]: Startup finished in 81ms.
Jul  8 12:12:42 guy systemd[1]: Started User Manager for UID 1011.
Jul  8 12:12:42 guy systemd[23224]: Started D-Bus User Message Bus.
Jul  8 12:12:42 guy dbus-daemon[23262]: [session uid=1011 pid=23262] AppArmor D-Bus mediation is enabled
Jul  8 12:12:43 guy kernel: [13698.675038] mtrr: no MTRR for d0000000,1000000 found
Jul  8 12:12:44 guy systemd[1]: Created slice User Slice of lightdm.
Jul  8 12:12:44 guy systemd[1]: Starting User Manager for UID 331...
Jul  8 12:12:44 guy systemd[1]: Started Session c156 of user lightdm.
Jul  8 12:12:44 guy systemd[23396]: Listening on GnuPG network certificate management daemon.
Jul  8 12:12:44 guy systemd[23396]: Listening on GnuPG cryptographic agent (ssh-agent emulation).
Jul  8 12:12:44 guy systemd[23396]: Reached target Paths.
Jul  8 12:12:44 guy systemd[23396]: Listening on GnuPG cryptographic agent and passphrase cache (access for web browsers).
Jul  8 12:12:44 guy systemd[23396]: Starting D-Bus User Message Bus Socket.
Jul  8 12:12:44 guy systemd[23396]: Listening on GnuPG cryptographic agent and passphrase cache (restricted).
Jul  8 12:12:44 guy systemd[23396]: Reached target Timers.
Jul  8 12:12:44 guy systemd[23396]: Listening on GnuPG cryptographic agent and passphrase cache.
Jul  8 12:12:44 guy systemd[23396]: Listening on D-Bus User Message Bus Socket.
Jul  8 12:12:44 guy systemd[23396]: Reached target Sockets.
Jul  8 12:12:44 guy systemd[23396]: Reached target Basic System.
Jul  8 12:12:44 guy systemd[23396]: Reached target Default.
Jul  8 12:12:44 guy systemd[23396]: Startup finished in 76ms.
Jul  8 12:12:44 guy systemd[1]: Started User Manager for UID 331.
Jul  8 12:12:44 guy systemd[23396]: Started D-Bus User Message Bus.
Jul  8 12:12:44 guy dbus-daemon[23424]: [session uid=331 pid=23424] AppArmor D-Bus mediation is enabled
Jul  8 12:12:44 guy dbus-daemon[23424]: [session uid=331 pid=23424] Activating via systemd: service name='org.a11y.Bus' unit='at-spi-dbus-bus.service' requested by ':1.0' (uid=331 pid=23414 comm="/usr/sbin/lightdm-gtk-greeter " label="unconfined")
Jul  8 12:12:44 guy systemd[23396]: Starting Accessibility services bus...
Jul  8 12:12:44 guy dbus-daemon[23424]: [session uid=331 pid=23424] Successfully activated service 'org.a11y.Bus'
Jul  8 12:12:44 guy systemd[23396]: Started Accessibility services bus.
Jul  8 12:12:44 guy at-spi-bus-launcher[23425]: dbus-daemon[23430]: Activating service name='org.a11y.atspi.Registry' requested by ':1.0' (uid=331 pid=23414 comm="/usr/sbin/lightdm-gtk-greeter " label="unconfined")
Jul  8 12:12:44 guy dbus-daemon[23424]: [session uid=331 pid=23424] Activating via systemd: service name='org.gtk.vfs.Daemon' unit='gvfs-daemon.service' requested by ':1.3' (uid=331 pid=23414 comm="/usr/sbin/lightdm-gtk-greeter " label="unconfined")
Jul  8 12:12:44 guy systemd[23396]: Starting Virtual filesystem service...
Jul  8 12:12:44 guy at-spi-bus-launcher[23425]: dbus-daemon[23430]: Successfully activated service 'org.a11y.atspi.Registry'
Jul  8 12:12:44 guy at-spi-bus-launcher[23425]: SpiRegistry daemon is running with well-known name - org.a11y.atspi.Registry
Jul  8 12:12:44 guy dbus-daemon[23424]: [session uid=331 pid=23424] Successfully activated service 'org.gtk.vfs.Daemon'
Jul  8 12:12:44 guy systemd[23396]: Started Virtual filesystem service.
Jul  8 12:12:52 guy systemd[1]: Stopping User Manager for UID 1011...
Jul  8 12:12:52 guy systemd[23224]: Stopping D-Bus User Message Bus...
Jul  8 12:12:52 guy systemd[23224]: Stopped target Default.
Jul  8 12:12:52 guy systemd[23224]: Stopped D-Bus User Message Bus.
Jul  8 12:12:52 guy systemd[23224]: Stopped target Basic System.
Jul  8 12:12:52 guy systemd[23224]: Stopped target Sockets.
Jul  8 12:12:52 guy systemd[23224]: Closed GnuPG network certificate management daemon.
Jul  8 12:12:52 guy systemd[23224]: Closed GnuPG cryptographic agent (ssh-agent emulation).
Jul  8 12:12:52 guy systemd[23224]: Closed GnuPG cryptographic agent and passphrase cache.
Jul  8 12:12:52 guy systemd[23224]: Closed GnuPG cryptographic agent and passphrase cache (restricted).
Jul  8 12:12:52 guy systemd[23224]: Closed GnuPG cryptographic agent and passphrase cache (access for web browsers).
Jul  8 12:12:52 guy systemd[23224]: Stopped target Paths.
Jul  8 12:12:52 guy systemd[23224]: Stopped target Timers.
Jul  8 12:12:52 guy systemd[23224]: Closed D-Bus User Message Bus Socket.
Jul  8 12:12:52 guy systemd[23224]: Reached target Shutdown.
Jul  8 12:12:52 guy systemd[23224]: Starting Exit the Session...
Jul  8 12:12:52 guy systemd[23224]: Received SIGRTMIN+24 from PID 23474 (kill).
Jul  8 12:12:52 guy systemd[1]: Stopped User Manager for UID 1011.
Jul  8 12:12:52 guy systemd[1]: Removed slice User Slice of rack.
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Fixed. The problem was that the login user's .ICEAuthority file was owned by root.

But I wonder how that happened. I checked earlier and I didn't see that situation, and since then I've done an update. The least hypothesis, of course, is that I made an error of some kind. I found the problem by re-checking the user's .xsession-errors, but I had already checked that.

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