Since the systemsettings dialog still shows OpenGL which cannot be changed to XRender, I will probably connect a monitor once and change it to XRender then.

Whatever it shows, please understand that - as mentioned:
COMPOSITING IS NOT SUPPORTED ON YOUR SETUP!
There's no composite extension, whatever the dialog shows is meaningless.

You *are* using either native or raster graphicssystem anyway - so one of the settings CANNOT make it worse.

Thanks, I understood that. Just I wanted to get back to a state in which I am actually to able to control and to know which system I am using (within the constraints given by the fact that I am using vnc).

I'm afraid the VNC server solution you are using appears to be incompatible with KDE - even with ARGB visuals disabled (although in theory the problem should not exist if ARGB visuals are disabled).

I suggest you try an alternative VNC server instead (such as RealVNC).