Hi Marco,

i don't know what causes this, but was able to generate a complete solid.
What i did was to create a G2 blend in MoI, then used the blended parts
and added at the end planar surfaces. after that i took the six surfaces
from the blend into Rhino OS X trial, created there a solid and imported
the solid back into MoI and booled all 3 three parts together. Attached
is a picture of the part which is now a solid in MoI.

Hope this works for you too.

Regards
Stefan