I'd say one very basic fix would be to check the polygon direction in C4d before exporting.

Therefore got to polygon mode and select all polys (ctrl+a).

Yellow polygons are pointing to you, blue ones away. To rearrange them go to Functions and "arrange normals" (I hope thats the exact name in the engl. version).

This can also be done by hitting [U] then [A].

If this solves the problem, so far so good. If not, it is necessary to get rid of those corner points with so many triangle-polys ending there. Therefore you can cut the outside edges of your model at as many points as you have on the inside hole. Then reconnect these points with the bridge-tool making quadrangles instead of triangles. This is a bit time consumting but gets way better results than these boolean mess.