I have seen this pop up when using II with Tweakscale. Seems that the counter in game does not properly take into effect the weight properly. It will throw a "Not a Number" and then makes the weight something absolutely absurd so that you cannot use the runway. When I run into that, I remove the part and grab it again and place it then it is fine. By the way, I know I said it before, but it needs to be said again, fantastic mod. This mod has been with me a while. I love the glowing blue on the tanks. Some people say it's overpowered, but no, it makes you think about building crafts in a bigger and better way. All my biggest craft, hovercrafts and satellites use II. This is one of my "essential" mods when I play.