Your object is not a manifold solid. It is a joined surface. Shapeways probably fixed the non-manifold issue by closing off that hole.

Do you remember seeing anything about non-manifold or "fixing" when you uploaded your model?

Here's a video where I make your object a solid. If you do this it should print fine.



So just to be clear, I selected only that surface. Then I ran SelectNakedEdges and chose loft (Without Caps), then joined the 2 parts to a solid.