hi eric thx for your help!
yesterday i found the reason. it was the scene scale. since i use a point light and the planet is about 5km away, the point light refuses to work properly at these distances (attenuation set to 6km, falloff turned off, 15 intensity) when i put the planet only 1km away, then the lighting will appear as also the shadows. but flying away with the cam will remove the lighting from the translucent object.
so i think i have to switch to a directional light that is placed at 0,0,0 and that will follow the ships/camera location.
this way i might have no scale issues as also access to the cascaded shadow map parameters.