I'm not an expert by any means, and I use Blender 2.79, but one thing I notice is that the naming seems off.
On your screenshot above Mermaid_L0 says 'Cube.001" while the other LOD's say "Untitled" followed by the copy number. If you look at the original version of the Mermaid you'll see that they are ALL showing "Untitled" followed by the copy number (other than the first copy)...
Strictly a guess, but perhaps that's where the error is coming from? Maybe that 'Cube.001" is causing the error since the editor then expects to see "Cube.002" next and instead sees a different name?
I checked a bunch of objectss I've made and on all of them the names remain the same. Again, I'm not an expert on Blender by any means, but that's something I noticed...