probably it has to be 0..1. now I have an idea what I could do wrong. but anyway, an example code from GPU Gems 3 works, as an interim solution, unfortunately shows some peter panning...

Edit: the depthmap max depth value was too high, if it is lowered everything works fine, but for really soft edged shadows, and for eliminating shadow fade-outs at terrain-object contact area, my original hack is still required...