@towo: I guess I found/located the problem. Seems related to some silly hardware configuration/combination problem.
I have the laptop attached to an HP-Dockingstation and had an additional Monitor connected via VGA to it which I switch on when needed. Worked always like a charm when needed with all resolutions I used.
So some weeks ago I got an bigger Monitor connected via DVI and was/am working since wih that when needed instead. No problems at all found until lately when I once again tried to use the virtual consoles.
Hadn't the slightest idea that it could have been related to that.
I found out/realized it when I tried to get lightdm working. It switched the screen completely black then, while sddm had frozen the screen.So during experimenting I disconnected the laptop from the docking station and the problem was gone. Reconnected the laptop to the docking station and disconnected that Monitor (the problem was even there when the Monitor was switched off) and the problem stayed gone.Reconnected the Monitor and Problem reappeared.
So knowing the cause yet I can avoid the problem. I might experiment with an cable with HDMI-Connector to see what effects this has.
So, sorry for the exitment and thanks to all trying to help me on that.
Regards
Reiner