It sure is not manifold. So it was made in 3 sections,
1. Ring for the chain
2. extruded circle, with indentation at it bottom
3. Embossed dragon design
Part 1 was boolean joined to part 2. Section 3 was not boolean joined to section 2. So the results were not 1 unified surface. Try just selecting parts 1,2,3 export as a stl, do not join meshes, upload to SW.
I worry that the indentation at the bottom, if the mesh is unified, might cause problems.
By the way, netfabb basic is a great help. but when you import a model there joined meshes causes problems.
Its just the logo area was joined with part 2, making both 1 mesh, along with part 1 being part of part 2.
That is what you call a oopsy. A simple mistake causing massive headaches