dies abruptly from, time to time

[Uqbar]

For some unknown reason, from time to time I am left without any window management.
No title bar, no window buttons, no ALT+TAB.
This is happening after a while (2+ hours) since reboot.
The only option I have is to switch to a local console (aka CTRL+ALT+F1) and run "sudo reboot" as "sudo /etc/init.d/sddm restart" doesn't work and seems to wait forever.
I am available to provide for more details, but I need to be directed on how to gather those.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: kwin-x11 4:5.5.5-0ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-21.37-lowlatency 4.4.6
Uname: Linux 4.4.0-21-lowlatency x86_64
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CurrentDesktop: KDE
Date: Thu Apr 28 07:58:23 2016
InstallationDate: Installed on 2016-04-22 (5 days ago)
InstallationMedia: Kubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: kwin
UpgradeStatus: No upgrade log present (probably fresh install)

[Uqbar (uqbar)]

I just checked there is no dumped core all over my system.

[Uqbar (uqbar)]

I am attaching the output of

qdbus org.kde.KWin /KWin org.kde.KWin.supportInformation