The UCX is a clone of the static mesh, to preserve the curvature and allow a player to jump parts of it as they run. As it's a convex shape, I hadn't expected a problem. The STL check shows no errors, so that's one thing down. Thanks for that option to try, though.
That's just a warning, I use the old version and have no issues. So what's the collision mesh look like, did you make sure it's completely closed? Add an STL check make sure you didn't miss an un-welded verts.