Hi Ingo - boolean operations can be difficult between pieces that have thin zones in them where different areas approach tangent to one another, like you have in this case in here:




The issue with such things is that surface on top sort of grazes over top of some of the same surface area as the other existing surface underneath it - that type of grazing situation means that when the surfaces are intersected with one another there is something more like a "zone of overlapping intersection" rather than a crisp well defined intersection.

In your case here you would be better off avoiding a boolean operation for this particular stage - booleans assume that some of the pieces involved may be partially sticking through each other and so the booleans do a surface/surface intersection step as part of their processing. But in your case here where your new surfaces connect up directly to existing edges, you can instead use Edit > Join to glue the pieces together. That should be more reliable for this particular situation because the Join command does not try to intersect surfaces with one another, it only tries to glue unjoined edges to one another. So in preparation for doing the join you would want to delete the corresponding faces from the main object underneath your new part, so that the main shape has unjoined edges in those areas, then select both parts and do Edit > Join to glue them together rather than doing a boolean operation.

It looks like one side of the model is not quite accurate enough to be joined currently though, you may need to rebuild your new surface on this side here because it does not come to the edge very accurately:





But basically once you got the pieces to be accurately coming to the same set of edges at that point you should use Edit > Join to glue the surfaces together rather than boolean union, particularly for this type of "grazing near tangent" type surface arrangements.

Hope this helps!

- Michael