"No solids" mystery

I’ve been working for a while on this, and all of the sudden I can no longer use the outer shell tool because it tells me some things are not solids. I’ve investigated every shape and carefully build them, I can’t seem to understand why this happened and why at that point of working on the file. I even tried copying groups that were not a problem before to a new file, but now everything seems to have a problem with the solid. I’ve attached the file below, any help would be much appreciated.

Roodtop 2.stl (151.7 KB)

The .stl isn’t much use for working out the solid issue, attach the .skp

Oh sorry, here’s the SKP: Roodtop 2.skp (738.1 KB)

In order for a component or group to be considered a solid, it must contain only raw geometry (edges and faces) and every edge must be share by exactly two faces. No more and no less. So no nested groups/components. No stray edges, no holes in surfaces, and no internal faces.