Screen lock does not work

[Otus]

Since upgrading to 18.04 the screen lock no longer works. The screen will not lock automatically despite the setting and even Ctrl+Alt+L and Super+L fail to do anything.

I've checked that

$ gsettings get org.gnome.desktop.lockdown disable-lock-screen
false

I'm not sure if lightdm is the correct package. I'm using lightdm, since gdm just gave me a black screen. But not sure which package should handle the lock.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: lightdm 1.26.0-0ubuntu1
ProcVersionSignature: Ubuntu 4.18.0-13.14~18.04.1-generic 4.18.17
Uname: Linux 4.18.0-13-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Sun Jan 20 13:00:21 2019
InstallationDate: Installed on 2012-11-10 (2261 days ago)
InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Release amd64 (20121017.5)
SourcePackage: lightdm
UpgradeStatus: Upgraded to bionic on 2019-01-17 (2 days ago)