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Sometimes, after running an update, if I lock my screen and then try to log in again from the lightdm greeter, it goes black, sits for a while, then puts me back at the login screen.

Ubuntu gets stuck in a login loop talks about ~/.Xauthority or /tmp permissions, but those seem OK.

/var/log/lightdm/lightdm.log shows things like this:

[+330172.81s] DEBUG: Seat seat0: Session authenticated, running command
[+330172.81s] DEBUG: Session pid=20966: Running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+330172.81s] DEBUG: Creating shared data directory /var/lib/lightdm-data/lightdm
[+330172.81s] DEBUG: Session pid=20966: Logging to /var/log/lightdm/x-3-greeter.log
[+330197.83s] DEBUG: Activating VT 10
[+330197.92s] DEBUG: Session pid=20966: Greeter connected version=1.16.7 resettable=false
[+330222.99s] DEBUG: Session pid=20966: Greeter start authentication for unhammer
[+330222.99s] DEBUG: Session pid=21031: Started with service 'lightdm', username 'unhammer'
[+330222.99s] DEBUG: Session pid=21031: Got 1 message(s) from PAM
[+330222.99s] DEBUG: Session pid=20966: Prompt greeter with 1 message(s)



[+330258.19s] DEBUG: Session pid=20966: Continue authentication
[+330258.22s] DEBUG: Session pid=21031: Authentication complete with return value 0: Success
[+330258.22s] DEBUG: Session pid=20966: Authenticate result for user unhammer: Success
[+330258.23s] DEBUG: Session pid=20966: User unhammer authorized
[+330258.23s] DEBUG: Session pid=20966: Greeter requests session xubuntu
[+330258.23s] DEBUG: Seat seat0: Returning to existing user session unhammer
[+330258.23s] DEBUG: Unlocking login1 session c44

[+330283.23s] WARNING: Error unlocking login1 session: GDBus.Error:org.freedesktop.DBus.Error.TimedOut: Failed to activate service 'org.freedesktop.login1': timed out
[+330283.23s] DEBUG: Activating VT 9
[+330283.36s] DEBUG: Seat seat0: Stopping greeter
[+330283.36s] DEBUG: Session pid=20966: Sending SIGTERM
[+330283.36s] DEBUG: Activating login1 session c44
[+330308.37s] WARNING: Error activating login1 session: Timeout was reached
[+330308.37s] DEBUG: Session pid=20966: Exited with return value 0
[+330308.37s] DEBUG: Seat seat0: Session stopped
[+330308.37s] DEBUG: Seat seat0: Greeter stopped, running session
[+330308.37s] DEBUG: Registering session with bus path /org/freedesktop/DisplayManager/Session11
[+330308.37s] CRITICAL: session_real_run: assertion '!session->priv->command_run' failed
[+330308.37s] DEBUG: Activating VT 10
[+330308.52s] DEBUG: Locking login1 session c44
[+330333.52s] WARNING: Error locking login1 session: GDBus.Error:org.freedesktop.DBus.Error.TimedOut: Failed to activate service 'org.freedesktop.login1': timed out
[+330333.52s] DEBUG: Session pid=21031: Exited with return value 0
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https://serverfault.com/a/742102/104546 gave a solution to a different question that also applied here: after an update, you may have to run sudo systemctl restart systemd-logind (ctrl+alt+f1 first to get to a plain terminal), then you can log in again.

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