I had the same issue, seeing a shadow instead of the STL. The solutions above did not work for me. I ran the inspector tool in meshmixer but it found no problems. I then used the Make Solid tool under Edit in meshmixer. In the object browser, you'll see the original STL and the new object that has solid added to it.
I made the original STL invisible and exported the new solid STL and, Al-hamdulillah, it worked perfectly fine.