Sky is reflected on interior surfaces

Try it out.

trung In Draft mode reflective surfaces reflect the skybox due a very simplified lighting.

If you want to avoid that, please use a higher rendering quality.

... strange - how is that possible even in draft mode - isn't there even a non-transparent face as a covering lid on that box?

Quote from EGIE

... strange - how is that possible even in draft mode - isn't there even a non-transparent face as a covering lid on that box?

What?

... nothing bad I just can't understand that the skybox is visible inside this entirely closed box or room - or isn't it closed?

EGIE as I've written, it is appearing in draft mode ONLY due very simplified lighting. Accurate reflections require a complex lighting calculation.