NavMesh may not be generated properly

HI there,
I hope to find some hints here: I get the following warning by Unity The NavMesh may not be generated properly as the vertical dimension of your Scene is too large. Try limiting the vertical size to less than … The scene can be seen here (https://www.youtube.com/watch?v=515ucouetgg) . The scene looks like this and the navmesh is way too high. Any help is highly appreciated. I hope to still use this level.
I played around with voxel size and agent height but no chance,
thank you very much