developer webapp does not show tree layer. why?

[CP_Leipzig]

Hello, I am using the latest developer edition for webapp builder.

I created a 3D WebApp and added a 3D scene from my Esri-Portal.

However, the 3D trees are not displayed, although they are active in the layer list.

Why??? What could be the problem?

(The trees are shown in the normal scene, without WebApp)

First picture = scene view

Second picture = Developer WebApp