Hi Lars, it looks like you might possibly have 2 pieces that are not joined together?

Try using Edit/Join to glue the pieces together into one piece.

When pieces are joined then Shell and Offset will do some work to make sure all the individual pieces offset towards the same direction. When you have 2 surfaces that are totally separate objects, it is easy for them to offset towards different sides.

Also if your object is not totally smooth where 2 open surfaces touch one another it can be difficult for Shell to thicken it properly especially in the area of the sharp corner. This can work better if you shell with a solid base object instead of with surfaces.

Sometimes you can use the Offset command to do a kind of manual construction of pieces which can help if Shell is having difficulties.

It is a bit difficult for me to give any more advice without seeing the actual .3dm model file - can you please post that as an attachment?

- Michael