sweet jesus those are some ugly boots, bad lukas, BAD!
I'm not 100 percent sure, but it looks like a badly compressed texture, maybe simply replacing the texture with a smoother gradient one would work? or maybe its a rendering thing, or something to do with shading. But to me, it looks like a texture. If it is a texture it should be fixable. If i knew where the texture for the sky was i would have a poke about, but i have no idea where it would be. sorry.