The default value was 0 minutes. I set it to one minute and it made no difference.
Incidentally, it seems that clicking on the "Switch user" button is enough to trigger the problem. I did this because I couldn't remember whether or not I'd added another user (I hadn't at that point). It's not necessary to actually log in as the other user. -- 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: Confirmed 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