Original User: Hellski
Im asking again, since I couldn't find a solution and the problem is becoming urgent!
What I did is to take my elevation data and created a tin from it. For the same points I created surface elevation data by adding a 0 as elevation and created a tin from that as well. Than I created a minimum bounding from those points using envelope (and RECTANGLE_BY_AREA with no difference). Than I used the extrude between tool with with elevation tin, surface tin and the minimum bounding as input feature class.
The result is, that two sides (north and east) are sloped as you can see in the pictures I attached in my posts.
I have no idea how this is possible ... please help.