render-problem suddenly green/purple

I have a problem. I'm working with Lightwave for some time, but am not an expert. And I have a problem with rendering. I work with Lightwave 9.0

I have a scene with about 10 objects and 3 lights, nothing too large or complex. I want to use "steamy-shader", so the objects that come into the light will cast shadows behind them, as in the mist.

This works.

The problem is the following: then 10 objects all use the same identical surface, and when I apply the same surfaces to the 8th and 9th item, the rendering suddenly starts off with full-green (instead of black), and the rendering becomes totally purple. This does not happen when I have 7 objects with the same surface and 3 objects that are "bald".

Any idea how to tackle this problem? I have done the rendering before, and that went very good! But now I don't know how to get rid of the green and purple?

Much thanks!

Ernst