I don't think this is Mate-specific. I'm seeing it with regular Gnome desktop.
This issue discusses the same problem. https://github.com/linuxmint/cinnamon-screensaver/issues/221 The seat name, which should be "seat0" is "seat0allow-timed-login" I have spent a couple of hours trawling through source code (online), but haven't reached a conclusion. I recorded this information when the problem occurs. >From journal: Sep 20 16:36:18 eliot gdm-launch-environment][5815]: pam_unix(gdm-launch-environment:session): session opened for user Debian-gdm by eliot(uid=0) Sep 20 16:36:18 eliot gdm-launch-environment][5815]: pam_systemd(gdm-launch-environment:session): Failed to create session: No seat 'seat0allow-time >From dbus-monitor: method call time=1505903552.055414 sender=:1.2159 -> destination=org.freedesktop.login1 serial=2 path=/org/freedesktop/login1; interface=org.freedes ktop.login1.Manager; member=CreateSession uint32 120 uint32 26182 string "gdm-launch-environment" string "x11" string "greeter" string "" string "seat0allow-timed-login" uint32 0 string "/dev/tty2" string "" boolean false string "" string "" array [ ] method call time=1505903552.055571 sender=:1.2 -> destination=org.freedesktop.DBus serial=11645 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=GetConnectionUnixUser string ":1.2159" method return time=1505903552.055612 sender=org.freedesktop.DBus -> destination=:1.2 serial=4321 reply_serial=11645 uint32 0 error time=1505903552.055699 sender=:1.2 -> destination=:1.2159 error_name=org.freedesktop.login1.NoSuchSeat reply_serial=2 string "No seat 'seat0allow-timed-login' known"
