In order to investigate this bug, I started to write down each time I switch user and whether it works.
The time information allowed to obtain time-bounded log extracts that focus on relevant parts using journalctl --since ... --until ... . After 3 "work" and one "fail" case, I observed a correlation, which might or might not be relevant. ## Logs when no bug, things work The following log sequence happens all 3 times things work okay: 2018-07-24T13:00:38.327651+0200 n551jk systemd[16387]: Startup finished in 26ms. 2018-07-24T13:00:38.327693+0200 n551jk systemd[1]: Started User Manager for UID 110. 2018-07-24T13:00:38.360247+0200 n551jk systemd[16387]: Started D-Bus User Message Bus. 2018-07-24T13:00:38.363237+0200 n551jk dbus-daemon[16407]: [session uid=110 pid=16407] AppArmor D-Bus mediation is enabled 2018-07-24T13:00:38.363739+0200 n551jk dbus-daemon[16407]: [session uid=110 pid=16407] Activating via systemd: service name='org.a11y.Bus' unit='at-spi-dbus-bus.service' requested by ':1.1' (uid=110 pid=16405 comm="/usr/sbin/lightdm-gtk-greeter " label="unconfined") 2018-07-24T13:00:38.364556+0200 n551jk systemd[16387]: Starting Accessibility services bus... 2018-07-24T13:00:38.368461+0200 n551jk dbus-daemon[16407]: [session uid=110 pid=16407] Successfully activated service 'org.a11y.Bus' 2018-07-24T13:00:38.368546+0200 n551jk systemd[16387]: Started Accessibility services bus. 2018-07-24T13:00:38.371337+0200 n551jk at-spi-bus-launcher[16408]: dbus-daemon[16413]: Activating service name='org.a11y.atspi.Registry' requested by ':1.0' (uid=110 pid=16405 comm="/usr/sbin/lightdm-gtk-greeter " label="unconfined") 2018-07-24T13:00:38.373982+0200 n551jk dbus-daemon[16407]: [session uid=110 pid=16407] Activating via systemd: service name='org.gtk.vfs.Daemon' unit='gvfs-daemon.service' requested by ':1.3' (uid=110 pid=16405 comm="/usr/sbin/lightdm-gtk-greeter " label="unconfined") 2018-07-24T13:00:38.374774+0200 n551jk at-spi-bus-launcher[16408]: dbus-daemon[16413]: Successfully activated service 'org.a11y.atspi.Registry' 2018-07-24T13:00:38.374918+0200 n551jk at-spi-bus-launcher[16408]: SpiRegistry daemon is running with well-known name - org.a11y.atspi.Registry 2018-07-24T13:00:38.375082+0200 n551jk systemd[16387]: Starting Virtual filesystem service... ## Logs when bug happens, things fail The bug happened once since I started to write down each operation, and it shows a difference: 2018-07-24T16:34:15.777621+0200 n551jk systemd[23296]: Startup finished in 23ms. 2018-07-24T16:34:15.777665+0200 n551jk systemd[1]: Started User Manager for UID 110. 2018-07-24T16:34:15.820108+0200 n551jk systemd[23296]: Started D-Bus User Message Bus. 2018-07-24T16:34:15.822959+0200 n551jk dbus-daemon[23316]: [session uid=110 pid=23316] AppArmor D-Bus mediation is enabled 2018-07-24T16:34:15.823499+0200 n551jk dbus-daemon[23316]: [session uid=110 pid=23316] Activating via systemd: service name='org.a11y.Bus' unit='at-spi-dbus-bus.service' requested by ':1.1' (uid=110 pid=23314 comm="/usr/sbin/lightdm-gtk-greeter " label="unconfined") 2018-07-24T16:34:15.824373+0200 n551jk systemd[23296]: Starting Accessibility services bus... 2018-07-24T16:34:15.828552+0200 n551jk dbus-daemon[23316]: [session uid=110 pid=23316] Successfully activated service 'org.a11y.Bus' 2018-07-24T16:34:15.828659+0200 n551jk systemd[23296]: Started Accessibility services bus. 2018-07-24T16:34:15.831689+0200 n551jk at-spi-bus-launcher[23317]: dbus-daemon[23322]: Activating service name='org.a11y.atspi.Registry' requested by ':1.0' (uid=110 pid=23314 comm="/usr/sbin/lightdm-gtk-greeter " label="unconfined") 2018-07-24T16:34:15.835154+0200 n551jk at-spi-bus-launcher[23317]: dbus-daemon[23322]: Successfully activated service 'org.a11y.atspi.Registry' 2018-07-24T16:34:15.835289+0200 n551jk at-spi-bus-launcher[23317]: SpiRegistry daemon is running with well-known name - org.a11y.atspi.Registry 2018-07-24T16:34:15.835578+0200 n551jk dbus-daemon[23316]: [session uid=110 pid=23316] Activating via systemd: service name='org.gtk.vfs.Daemon' unit='gvfs-daemon.service' requested by ':1.4' (uid=110 pid=23314 comm="/usr/sbin/lightdm-gtk-greeter " label="unconfined") 2018-07-24T16:34:15.836417+0200 n551jk systemd[23296]: Starting Virtual filesystem service... ## Difference between "work" and "fail" All 3 lines mentioning at-spi-bus-launcher come *before* : dbus-daemon ... Activating via systemd: service name='org.gtk.vfs.Daemon'. I don't know it this is relevant at this point in time. No other salient difference in logs. Logs attached. ** Attachment added: "Example log when things work." https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/1776475/+attachment/5167260/+files/2018y07m24d-12h42m24s_activite_je_passe_sur_session_stephane_avec_export_xdg_seat_path__org_freedesktop_displaymanager_seat0___dm_tool_switch_to_user_stephane____exit.timestamped.log.txt -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to lightdm in Ubuntu. https://bugs.launchpad.net/bugs/1776475 Title: Unlocking existing session often requires several attempts. Status in lightdm package in Ubuntu: New Bug description: # Context * Computer with several X sessions opened. * Users switching from one opened session to another. * Lightdm opens new greeter, wait for typing password. * User types password. * Password is accepted. * System closes the current greeter (screen gets black for an instant). # Expected * User gets back to their already opened session. # Observed * System immediately opens another greeter. * User has to type password again. # Reproducible * Not always. At times, often, at other times, rare. # Additional information Users switch session via dm-tool switch-to-user $OTHERUSER but I guess bug appears with other means (like menu item like "lock screen", etc). # Information requested lsb_release -rd Description: Ubuntu 18.04 LTS Release: 18.04 LC_ALL=C apt-cache policy lightdm lightdm: Installed: 1.26.0-0ubuntu1 Candidate: 1.26.0-0ubuntu1 Version table: *** 1.26.0-0ubuntu1 500 500 http://fr.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages 100 /var/lib/dpkg/status ProblemType: Bug DistroRelease: Ubuntu 18.04 Package: lightdm 1.26.0-0ubuntu1 ProcVersionSignature: Ubuntu 4.15.0-23.25-generic 4.15.18 Uname: Linux 4.15.0-23-generic x86_64 ApportVersion: 2.20.9-0ubuntu7.2 Architecture: amd64 Date: Tue Jun 12 14:14:52 2018 ExecutablePath: /usr/sbin/lightdm InstallationDate: Installed on 2018-05-25 (18 days ago) InstallationMedia: Xubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426) ProcEnviron: LANG=fr_FR.UTF-8 PATH=(custom, no user) SourcePackage: lightdm UpgradeStatus: No upgrade log present (probably fresh install) modified.conffile..etc.logrotate.d.lightdm: [modified] mtime.conffile..etc.logrotate.d.lightdm: 2018-05-25T06:19:22.081830 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/1776475/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp