Very low quality shadows when scene modified.

[lucmska]

I've worked on this for quite a long time. The client wants to see every single change. When I rendered it in the beginig I had images of an acceptable quality.

The more I change the scene (adding objects or replacing materials) the worse the quality gets. After I added the brown object, I don't get shadows anymore, unless I remove the floor from my scene. I replaced the brown object with one I have made myself, but it didn't help.

All the materials are Arch&design. Renderer: mental ray.

How to fix it?

Please help!