The screen locker is broken and unlocking is not possible anymore

[tavasti]

Every time I lock screen, screen locker dies and screen shows message:

The screen locker is broken and unlocking is not possible anymore.
In order to unlock switch to a virtual terminal (e.g Ctrl+Alt+F2),
log in and execute the command:

loginctl unlock-sessions

Afterwards switch back to the running session (Ctrl+Alt+F7)

ProblemType: Crash
DistroRelease: Ubuntu 16.04
Package: libkscreenlocker5 5.5.5-0ubuntu1
ProcVersionSignature: Ubuntu 4.10.0-40.44~16.04.1-generic 4.10.17
Uname: Linux 4.10.0-40-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.13
Architecture: amd64
CurrentDesktop: KDE
Date: Sun Nov 26 15:58:14 2017
ExecutablePath: /usr/lib/x86_64-linux-gnu/libexec/kscreenlocker_greet
InstallationDate: Installed on 2017-11-26 (0 days ago)
InstallationMedia: Kubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
ProcCmdline: /usr/lib/x86_64-linux-gnu/libexec/kscreenlocker_greet --immediateLock --graceTime 5000 --ksldfd 31
Signal: 6
SourcePackage: kscreenlocker
StacktraceTop:
 __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:54
 __GI_abort () at abort.c:89
 QMessageLogger::fatal(char const*, ...) const () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
 QSGRenderLoop::handleContextCreationFailure(QQuickWindow*, bool) () from /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
 ?? () from /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
Title: kscreenlocker_greet crashed with SIGABRT in __GI_raise()
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip lpadmin plugdev sudo

[Apport retracing service (apport)]

Thank you for taking the time to report this crash and helping to make this software better. This particular crash has already been reported and is a duplicate of bug #1445447, so is being marked as such. Please look at the other bug report to see if there is any missing information that you can provide, or to see if there is a workaround for the bug. Additionally, any further discussion regarding the bug should occur in the other report. Please continue to report any other bugs you may find.