Hi Andreas, if you select all the edges of an open boundary then you can use Delete to remove that trim boundary to recover the underlying surface.

There are some examples here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4
http://moi3d.com/forum/index.php?webtag=MOI&msg=9671.5

If you're still stuck please post your .3dm model file or e-mail it to me at moi@moi3d.com

- Michael