Error when giving thickness to subd surface

Hello! I want to give thickness of 2-3 mm to this subd surface for printing, but it keeps getting defects when I do so, why is that happening and how to fix it?

sky bridge to print.3dm (808.6 KB)

Looks pretty good in Rhino 8 apart from this opening where I selected a few faces…

What errors can you see?

like the picture I attached has defects in the openings… can you send me the file with the offset you did?

I deleted some small parts.

sky bridge to print mrtn.3dm (1011.1 KB)

Thank you Martin! much appreciated