At the same time It works well in regular 2.8 version of Blender. The problem occurred in 2.81 E-cycles build.

Not sure. I didn't try Blender 2.81. Is the problem only in e-cycles 2.81 or also with the daily build of 2.81?

Thanks for the prompt feedback Scrubs. Yes, it turned out that the problem persist only in e-cycles 2.81 RTX build. Will try to reach the build's creator for a fix.

Ok. It was working with an old version of e-cycles 2.80 which I bought at the time.

Just to close up the subject. Got the feeback from E-Cycles creator: "The "mb_normal" node group (maybe others too) has a bevel node, which is not supported by NVidia Optix for now. You can ask the add-on developer to make an "Optix materials" option to it's importer, that would use the appropriate node groups".

The bevel node is a key component of the material. That is what makes your scene look much more realistic. If really you want to render with optix instead of CUDA, just mute the bevel node in mb_normal.

Yep, that's what I did. Thanks Scrubs.