Thanks Matt, I scaled the displacement down to 0.1. When I turn on the cast shadows in the fill light now, it renders correctly now, not a gray (shadowed) landscape but one with the intended colors! Still it's not easy to find the right settings in the fill lights to come up with the right appearance. But I keep trying...
Working with the fill lights setup and reading around on the forums these days (Oshyan's fill lights, Moodflow's), I tend to think it's better at the moment to turn off GI anyhow. It won't be as easy as that I think, but anyway I see some advantages:
-a great (very great!) decrease in render times
-more possiblitities to adjust lightning
-with GI turned on, a small (and I mean real small ) change in camera point of view will change the lighting in the final render dramatically; even so: crop render with GI turned on differs a lot from the same part in a full render of the whole picture, etc. So GI off means more "render certainty", you know what you will get...
There must (?) be a loss when turning GI off I guess; shadows look much harder now I believe, but maybe that means more tweaking in the fill lights settings...(?) I have to practice a lot I guess, but
I believe it's worth it: never saw a render engine that could produce such picturesque and crispy renders...!