Changing 3d Models in Vuforia Core Sample

Hi,

I am currently tinkering around with your GroundPlane Sample Scene which i Imported along with all of the rest of the Core Samples. To get a feel of how to work with it. The Samples work beautifully in Instant Play and on my Samsung S8. However when i try to change the 3d Models instead of the Camera feed i get a blackscreen on which the UI is rendered on both instant play and when running it on the Phone. This probably happens because i don't know how to do it correctly, if you could tell me what I am doing wrong or how to do it correctly i would appreciate it.

What I am doing:

1. Importing Core Samples & adding my License Key
2. Opening Groundplane scene (running now works as one would expect)
3. Adding my 3D model as a child of the Anchor_Placement node in the scene hierarchy (running now shows both my 3d Model and the Chair Model as one would expect)
4. Removing the Chair from the Hierarchy

running after the 4th step shows the UI rendered on a black background instead of the camera feed. I can still infer from the UI that it is tracking the ground because the icons change when i point my camera towards the instant play target.

TL;DR: what am i doing wrong?

greetings and thanks for your help Patrick