****If that reads "Joined Srf" instead of "Solid" there, then that means your object is not fully closed.*****

Hello Michael,

that's a useful Info – Is there a way to recreate Solid from Surfaces for STL creation?
I guess, the fixing, I did afterwards in Netfabb wouldn't have been necessary if it was a solid. There were some "degenerated faces" -as they call it in NF- detected and I don't get where they came from, since it is a simple extrude – But no warnings, it wouldn't be printable: Neither in the software, nor from shapeways, so I guess It'll turn out alright.

Regards,
Sebastian