Go to the options tab labeled "Window Capturing 1" and uncheck the box marked "Try to capture Aero Glass transparency effects".
See if that solves it.
Thank you for your answer. I found that one earlier but checked anyway.
Here are the results:
"Aero Gloss..." unchecked: black interior
Window capturing 2, first check box "Try to detect..." unchecked: black interior
Window capturing 2, below, "Do not try ..." checked: Well, folks, I can't believe it: regular screenshot of the whole window!!!!!!
Double checked with both items checked in window capturing 2: Also okay screenshot.
It looks very much like that option "Do not try to use PrtWindow() API ..." is the key, at least for my current problem.
You think it would help anybody else, maybe?