Thank you, Anton!
I used an old plugin by Rick Wilson "fixAspectRatio.rb", but it does not seem to work in SketchUp 2017.
The problem is, my original scene has no gray bars. After using your line there are dramatically changes in FOV and focal length.
But there are no gray surfaces in the exported 2D-image - a first success!
Edit:
One possible solution is to restore the old FOV values in the menu Camera Advanced (plugin tt_camera) after entering your code line into the Ruby Console.