Try this:
Right click on Orbiter's icon to show its properties, go for the "Compatibility" tab, set a checkmark at "Deactivate scaling for high DPI-levels".
By default Winows enlarges graphics at high DPI levels, to avoid unreadable small text. Unfortunately this seems to extend the 'logical' screen beyond the physical size of the screen for some graphics cards when in full-screen mode.
That worked, thanks! :thumbup: