I have a sighting of this. Interested in helping to diagnose it... but someone more expert than I about PAM and systemd etc will have to tell me what to type to generate any useful logging!
In fact I realise I've seen it twice now: a few weeks ago I was using the 32bit Debian9 installer to nuke the HD on an old long unused Thinkpad T22 I was giving away. Out of interest I thought I'd do a whole install. When gdm3 didn't come up with anything but the dark-grey textured "wallpaper" (but no dialog prompting for who's logging in) I thought the ageing Thinkpad's graphics HW just couldn't cope and switched to lightdm, which worked fine. With hindsight I realize it was probably this issue. And now just this morning I'm doing a 64bit Debian9 install on a shiny new 7700K system. And see the same problem. A bit of googling led to this issue. FWIW, a few things: Output of systemctl status gdm3: gdm.service - GNOME Display Manager Loaded: loaded (/lib/systemd/system/gdm.service; static; vendor preset: enabled) Active: active (running) since Tue 2017-09-26 15:18:54 BST; 11min ago Process: 916 ExecStartPre=/usr/share/gdm/generate-config (code=exited, status=0/SUCCESS) Process: 912 ExecStartPre=/bin/sh -c [ "$(cat /etc/X11/default-display-manager 2>/dev/null)" = "/usr/sbin/gdm3" ] (code=exited, status=0/SUCCESS) Main PID: 925 (gdm3) Tasks: 3 (limit: 4915) CGroup: /system.slice/gdm.service └─925 /usr/sbin/gdm3 Sep 26 15:18:54 everest systemd[1]: Starting GNOME Display Manager... Sep 26 15:18:54 everest systemd[1]: Started GNOME Display Manager. Sep 26 15:18:54 everest gdm-launch-environment][932]: pam_unix(gdm-launch-environment:session): session opened for user Debian-gdm by (uid=0) Tail end of /var/log/syslog after boot: Sep 26 15:34:25 everest systemd[1]: Started Network Manager Wait Online. Sep 26 15:34:25 everest systemd[1]: Reached target Network is Online. Sep 26 15:34:25 everest systemd[1]: apt-daily.timer: Adding 1h 17min 31.447421s random time. Sep 26 15:34:25 everest systemd[1]: Started Daily apt download activities. Sep 26 15:34:25 everest systemd[1]: apt-daily-upgrade.timer: Adding 29min 15.620376s random time. Sep 26 15:34:25 everest systemd[1]: Started Daily apt upgrade and clean activities. Sep 26 15:34:25 everest systemd[1]: Reached target Timers. Sep 26 15:34:25 everest systemd[1]: Starting keep memory of all UPnP devices that announced themselves... Sep 26 15:34:25 everest systemd[1]: Started keep memory of all UPnP devices that announced themselves. Sep 26 15:34:25 everest systemd[1]: Reached target Multi-User System. Sep 26 15:34:25 everest systemd[1]: Reached target Graphical Interface. Sep 26 15:34:25 everest systemd[1]: Starting Update UTMP about System Runlevel Changes... Sep 26 15:34:25 everest systemd[1]: Started Update UTMP about System Runlevel Changes. Sep 26 15:34:25 everest systemd[1]: Startup finished in 2.818s (kernel) + 9.195s (userspace) = 12.014s. Sep 26 15:34:47 everest systemd-timesyncd[529]: Synchronized to time server 176.58.109.199:123 (2.debian.pool.ntp.org). Sep 26 15:35:33 everest systemd[1]: Created slice User Slice of local. Sep 26 15:35:33 everest systemd[1]: Starting User Manager for UID 1000... Sep 26 15:35:33 everest systemd[1]: Started Session 2 of user local. Sep 26 15:35:33 everest systemd[856]: Reached target Paths. Sep 26 15:35:33 everest systemd[856]: Listening on GnuPG cryptographic agent (access for web browsers). Sep 26 15:35:33 everest systemd[856]: Listening on GnuPG cryptographic agent (ssh-agent emulation). Sep 26 15:35:33 everest systemd[856]: Listening on GnuPG cryptographic agent and passphrase cache. Sep 26 15:35:33 everest systemd[856]: Reached target Timers. Sep 26 15:35:33 everest systemd[856]: Starting D-Bus User Message Bus Socket. Sep 26 15:35:33 everest systemd[856]: Listening on GnuPG cryptographic agent and passphrase cache (restricted). Sep 26 15:35:33 everest systemd[856]: Listening on D-Bus User Message Bus Socket. Sep 26 15:35:33 everest systemd[856]: Reached target Sockets. Sep 26 15:35:33 everest systemd[856]: Reached target Basic System. Sep 26 15:35:33 everest systemd[856]: Reached target Default. Sep 26 15:35:33 everest systemd[856]: Startup finished in 10ms. Sep 26 15:35:33 everest systemd[1]: Started User Manager for UID 1000. And then after a systemctl restart gdm3: Sep 26 15:37:49 everest systemd[1]: Stopping GNOME Display Manager... Sep 26 15:37:49 everest gnome-shell[671]: Connection to xwayland lost Sep 26 15:37:49 everest org.a11y.atspi.Registry[707]: XIO: fatal IO error 11 (Resource temporarily unavailable) on X server ":1024" Sep 26 15:37:49 everest org.a11y.atspi.Registry[707]: after 21 requests (21 known processed) with 0 events remaining. Sep 26 15:37:49 everest kernel: [ 216.033849] traps: gnome-shell[671] trap int3 ip:7f8f62c41261 sp:7ffd06d36bb0 error:0 Sep 26 15:37:49 everest kernel: [ 216.033861] in libglib-2.0.so.0.5000.3[7f8f62bf1000+112000] Sep 26 15:37:49 everest gdm3: Child process -659 was already dead. Sep 26 15:37:51 everest systemd[1]: Stopping User Manager for UID 117... Sep 26 15:37:51 everest systemd[654]: Stopping Sound Service... Sep 26 15:37:51 everest systemd[654]: Stopped target Default. Sep 26 15:37:51 everest systemd[654]: Stopping Accessibility services bus... Sep 26 15:37:51 everest systemd[654]: Stopping D-Bus User Message Bus... Sep 26 15:37:51 everest systemd[654]: Stopped D-Bus User Message Bus. Sep 26 15:37:51 everest systemd[654]: Stopped Accessibility services bus. Sep 26 15:37:51 everest systemd[654]: Stopped Sound Service. Sep 26 15:37:51 everest systemd[654]: Stopped target Basic System. Sep 26 15:37:51 everest systemd[654]: Stopped target Sockets. Sep 26 15:37:51 everest systemd[654]: Closed GnuPG cryptographic agent (access for web browsers). Sep 26 15:37:51 everest systemd[654]: Closed GnuPG cryptographic agent and passphrase cache. Sep 26 15:37:51 everest systemd[654]: Closed GnuPG cryptographic agent and passphrase cache (restricted). Sep 26 15:37:51 everest systemd[654]: Closed D-Bus User Message Bus Socket. Sep 26 15:37:51 everest gdm3: GLib: g_hash_table_find: assertion 'version == hash_table->version' failed Sep 26 15:37:51 everest systemd[654]: Closed GnuPG cryptographic agent (ssh-agent emulation). Sep 26 15:37:51 everest systemd[654]: Stopped target Timers. Sep 26 15:37:51 everest systemd[654]: Stopped target Paths. Sep 26 15:37:51 everest systemd[654]: Closed Sound System. Sep 26 15:37:51 everest systemd[654]: Reached target Shutdown. Sep 26 15:37:51 everest systemd[654]: Starting Exit the Session... Sep 26 15:37:51 everest systemd[1]: Stopped GNOME Display Manager. Sep 26 15:37:51 everest systemd[654]: Received SIGRTMIN+24 from PID 890 (kill). Sep 26 15:37:51 everest systemd[1]: Starting GNOME Display Manager... Sep 26 15:37:51 everest systemd[1]: Stopped User Manager for UID 117. Sep 26 15:37:51 everest systemd[1]: Removed slice User Slice of Debian-gdm. Sep 26 15:37:51 everest systemd[1]: Started GNOME Display Manager. Sep 26 15:37:51 everest systemd[1]: Created slice User Slice of Debian-gdm. Sep 26 15:37:51 everest systemd[1]: Starting User Manager for UID 117... Sep 26 15:37:51 everest systemd[1]: Started Session c2 of user Debian-gdm. Sep 26 15:37:51 everest systemd[912]: Starting D-Bus User Message Bus Socket. Sep 26 15:37:51 everest systemd[912]: Reached target Paths. Sep 26 15:37:51 everest systemd[912]: Listening on GnuPG cryptographic agent and passphrase cache. Sep 26 15:37:51 everest systemd[912]: Listening on GnuPG cryptographic agent (ssh-agent emulation). Sep 26 15:37:51 everest systemd[912]: Listening on Sound System. Sep 26 15:37:51 everest systemd[912]: Listening on GnuPG cryptographic agent (access for web browsers). Sep 26 15:37:51 everest systemd[912]: Reached target Timers. Sep 26 15:37:51 everest systemd[912]: Listening on GnuPG cryptographic agent and passphrase cache (restricted). Sep 26 15:37:51 everest systemd[912]: Listening on D-Bus User Message Bus Socket. Sep 26 15:37:51 everest systemd[912]: Reached target Sockets. Sep 26 15:37:51 everest systemd[912]: Reached target Basic System. Sep 26 15:37:51 everest systemd[912]: Reached target Default. Sep 26 15:37:51 everest systemd[912]: Startup finished in 24ms. Sep 26 15:37:51 everest systemd[1]: Started User Manager for UID 117. Sep 26 15:37:51 everest systemd[912]: Started D-Bus User Message Bus. Sep 26 15:37:51 everest org.gnome.Shell.desktop[929]: glamor: EGL version 1.4 (DRI2): Sep 26 15:37:52 everest dbus-daemon[919]: Activating via systemd: service name='org.a11y.Bus' unit='at-spi-dbus-bus.service' Sep 26 15:37:52 everest systemd[912]: Starting Accessibility services bus... Sep 26 15:37:52 everest dbus-daemon[919]: Successfully activated service 'org.a11y.Bus' Sep 26 15:37:52 everest systemd[912]: Started Accessibility services bus. Sep 26 15:37:52 everest at-spi-bus-launcher[946]: Activating service name='org.a11y.atspi.Registry' Sep 26 15:37:52 everest at-spi-bus-launcher[946]: Successfully activated service 'org.a11y.atspi.Registry' Sep 26 15:37:52 everest org.a11y.atspi.Registry[951]: SpiRegistry daemon is running with well-known name - org.a11y.atspi.Registry Sep 26 15:37:52 everest systemd[912]: Starting Sound Service... Sep 26 15:37:52 everest systemd[912]: Started Sound Service. Sep 26 15:37:52 everest dbus[568]: [system] Activating via systemd: service name='org.freedesktop.locale1' unit='dbus-org.freedesktop.locale1.service' Sep 26 15:37:52 everest systemd[1]: Starting Locale Service... Sep 26 15:37:52 everest gnome-shell[929]: JS LOG: Failed to launch ibus-daemon: Failed to execute child process "ibus-daemon" (No such file or directory) Sep 26 15:37:52 everest dbus[568]: [system] Activating via systemd: service name='org.freedesktop.GeoClue2' unit='geoclue.service' Sep 26 15:37:52 everest systemd[1]: Starting Location Lookup Service... Sep 26 15:37:52 everest dbus[568]: [system] Successfully activated service 'org.freedesktop.GeoClue2' Sep 26 15:37:52 everest systemd[1]: Started Location Lookup Service. Sep 26 15:37:52 everest dbus[568]: [system] Activating via systemd: service name='org.freedesktop.realmd' unit='realmd.service' Sep 26 15:37:52 everest systemd[1]: Starting Realm and Domain Configuration... Sep 26 15:37:52 everest dbus[568]: [system] Successfully activated service 'org.freedesktop.realmd' Sep 26 15:37:52 everest systemd[1]: Started Realm and Domain Configuration. Sep 26 15:37:52 everest gnome-session-binary[921]: Entering running state Sep 26 15:37:52 everest xbrlapi.desktop[987]: openConnection: connect: No such file or directory Sep 26 15:37:52 everest xbrlapi.desktop[987]: cannot connect to braille devices daemon brltty at :0 Sep 26 15:37:52 everest dbus[568]: [system] Activating via systemd: service name='org.freedesktop.hostname1' unit='dbus-org.freedesktop.hos tname1.service' Sep 26 15:37:52 everest systemd[1]: Starting Hostname Service... Sep 26 15:37:52 everest dbus[568]: [system] Successfully activated service 'org.freedesktop.locale1' Sep 26 15:37:52 everest systemd[1]: Started Locale Service. Sep 26 15:37:52 everest dbus[568]: [system] Successfully activated service 'org.freedesktop.hostname1' Sep 26 15:37:52 everest systemd[1]: Started Hostname Service. Sep 26 15:37:52 everest gnome-settings-[969]: g_task_return_error: assertion 'error != NULL' failed I've tried the various solutions proposed here, but no improvement. FWIW, output of apt-show-versions | grep 'pam\|systemd': libpam-cap:amd64/stretch 1:2.25-1 uptodate libpam-gnome-keyring:amd64/stretch 3.20.0-3 uptodate libpam-modules:amd64/stretch 1.1.8-3.6 uptodate libpam-modules-bin:amd64/stretch 1.1.8-3.6 uptodate libpam-runtime:all/stretch 1.1.8-3.6 uptodate libpam-systemd:amd64/stretch 232-25+deb9u1 uptodate libpam0g:amd64/stretch 1.1.8-3.6 uptodate libsystemd0:amd64/stretch 232-25+deb9u1 uptodate systemd:amd64/stretch 232-25+deb9u1 uptodate systemd-sysv:amd64/stretch 232-25+deb9u1 uptodate Install was a graphical expert install. Nothing unusual except RAID1-ing a pair of disks, and also selecting ssh-server in tasksel (precisely because I like to have a way into the machine if this sort of graphics problem happens). But let me know what else you want/need to see/try... For now I'm switching to lightdm (which does put up a useful login prompt) but I can easily hop back to gdm3 to try things. Tim