@cadfather said:

it looks so weird that it doesn't seem a vray problem (at any rate i never experienced anything this bad, and when it fails, a restart or importing the model into a new empty file does the job).

Hi CadFather
You are right,it wasn´t a Vray problem 😡 ...In fact after my post I start a new sketchup file within the same open session, rendered it with the existing vray settings and magic..it works like a charm.
Conclusion - It was a Sketchup problem
Then I visit the book http://www.aidanchopra.com/book-info and made a complete new set-up of shadows and geo-positioning of model and after a few tweaks the model is back rendering without problems - see sample.
Thanks for your input

Stairs - Shadows and wall-paintings.jpg