Hi Clemens,
You should only add the mesh layer to the SLF (no boundary polygon of any sort is necessary), the outside nodes constitute the boundary. As for the boundary file, it is not possible (yet) to create that file with Blue Kenue. I hope this will be a new feature. The procedure in Fudaa-prepro explained by Sven works for me. If that stil doesn't work, you might want to try to check the quality of your mesh in the Mesh Analysis tabs and see where and what node 7914 looks like...
While we're at it, do you know of anyone that is actually working with little-endian selafin files? If not, I think it might be interesting for BK to save as big-endian by default. It would probably be a time saver for some of us that keep forgetting to select the option.
j