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.
