Shadow of objects behind objects with disabled shadows absent

[3d-laboratory]

I often use a box to assign a background on it. To prevent, that this box casts shadows on the scene I switch the "Cast shadows" off in the objects properties. If now another object behind that box is lit by sunlight it casts no shadows itself. I think this is caused because it is in a position where it would get theoretically shadows from the "backgrond box". Is there another possibility to tell an object not to cast shadows?

[Morne]

Have you tried setting the box to also NOT "receive shadows"?

[hermit.crab]

Not sure if I am understanding correctly, but the scenario seems to work on a simple scene - the object behind the box is lit on one side and casts a shadow. Could you share a screencap of the issue?

[3d-laboratory]

Sorry for my late answer, I stick in heavy work. When the deadline is over I will post a screenshot here. But it seems to work what Morne wrote: Also switch of "receive shadows".

[3d-laboratory]

Now the screenshots and what I found: After opening the scene the box (the bluish thing on the left) casts no shadow as it should. Receive shadows is on, cast shadows is off for the box. But also the trees in the shadow area of the box cast no shadows, while the building casts shadows. The trees are inserted with Forest Pack. To illustrate how the box casts shadow I switched on "Cast shadows" in the boxes properties. Third I swtiched off "Receive shadows" and, abracadabra, the shadows of the trees are back again. And now I can switch off/switch on "Receive shadows" as long as I want, the shadows of the trees are shown corectly. Conclusion: I had to switch "Receive shadows" once and all is working normal. But I have to do it every time I start the scene. The issue is connected to the fact, that the trees are either XRef objects or the fact, that they are scattered with Forest Pack. Other objects in the scene casts shadows normal in every case.