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I'm currently getting the same message with a .3dn file that I created for the game RoboWarrior. I'm using version 2.1.0, and it's happened twice now when starting from scratch. I tried starting from scratch and just changing a couple of things, but it seemed to load fine, so I'm guessing that something I've done both times is causing the issue. Here is the .3dn file:

https://www.dropbox.com/s/h32n724hw0kn518/Robo%20Warrior%20%28U%29.3dn?dl=0