Bug#929834: Found a workaround which also solves other oddities
I did try the different workarounds in Buster like "xset s off",
disabling light-locker so it wouldn't start, tried kernel 5.3. All these
improved the situations with "black screen", but finally still some
situations where the monitor should awake from DPMS state remained and I
had to switch virtual console (Ctrl+Alf+F1/F7) randomly every few days.
Now I discovered this posting from Dai_trying:
http://forums.debian.net/viewtopic.php?f=10&t=142383#p700796
and generated a /etc/X11/xorg.conf as described
service lightdm stop
X -configure
cp /root/xorg.conf.new /etc/X11/xorg.conf
And that really works without any glitches for over a week so far.
Additionally it also solved another bug which I reported on 19. March 2019:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=893559
It caused solaar (tool to manage Logitech Unifying Receiver) failing to
start in xfce4 when set up for automatic start in session. Now it starts
reliable iconised in the xfce4-panel.
So, my guess is that this nasty bug is actually caused not caused by
light-locker, but instead by xorg-server which either looses the mouse
or fails to (re)start with correct configuration. With said xorg.conf
all this is gone.
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