Little Update:
I have 2 Projects:
Project A: Uses ARDK 1.3.1 and the Examples and Mock environment for it
Project B: Uses ARDK 2.0.0 and the Examples and Mock environment for it
In each one I’ve build a scene only to visualize Meshing. (Without using any prefabs, templates or similar). So that i have now in each project the exact same scene with exact same Hierarchy (same GameObjects, same Components).
Project A shows the ground and finds everything immediately and the mesh is very fine. Project B only finds walls and ceiling, is (subjectively) very slow and the mesh is extremely “rough” (e.g. you can’t regognize Bottles on a desk or similar).
When i bring the scene from Project B into Project A, so that there are really the exact same scenes in the same Project, it still does not show the ground.
But when i go the other way and bring the scene, which was originally created in Project A, into Project B, i can see the ground in a project using ARDK 2.0.0 
It is really confusing, because it seems then in the end not to be a problem with the ARDK 2.0.0 itself, but with anything else (just what???)
So, this is at least a workaround, to be able to use Ground Detection on a mesh in ARDK. Maybe it helps anybody, too. Or it helps you for troubleshooting @Gilberto
But really wonder why it works, and would be nice, if it could be fixed, so that you can create a scene also directly in the project, which you really use 