Hey Klaus,
Out of frustration with cloud-on-cloud shadow system I made a few tests. I also tried 2D shadow map hoping this may solve my problem, but I guess you could use it.
I would say that 2D shadow map has influence on both terrain and clouds though, indeed, it may not be very profound. Probably the blur setting has influence on it. Anyway, have a look at these pictures and the areas marked.
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