Rendering problem

New try on the Stirling Single. Totally redesigned the smokebox area and raised the boiler which I think looks better than my last try. I know the cylinders stick out a bit too much, but that's something I'll have to compromise.

Now the real problem will be building this in real life.

 

And you can see another problem I have - rendering just won't work out and I don't understand what's the problem. Anyone any idea what might cause the error?

Done