When I attempt to shutdown with the gear icon, the login screen always comes back. When I Ctrl+Alt+F1 into a PTY, logon and run shutdown 0, it goes into single-user mode but remains there. Subsequent attempts produce the same result.

I grepped /var/log/* (as root) for "shutdown" but found nothing. man shutdown only mentions that it writes a record to /var/log/wtmp and utmp which are binary files, and which doesn't sound like it will help me much in this case since the docs indicates they appear to be about tracking logins and logouts, not attempts to shut down the system.

I can't write down all the messages that it dumps on the screen. How else can I debug this? Doesn't shutdown leave a trace somewhere?