Thanks for the support!
Actually, trying to describe what happens wrong (the sequence of my steps) I got it right now! Thanks!!!
I did not keep the "G" AND left mouse button pressed while moving the item onto the other inventory
If I release one of these keys/buttons I lose the control and end behind the transparent inventory.
Looking into the manual and Igor's note further up, I see now that you have to "maintain "G"", one just has to read and follow Maybe the mouse button should also be mentioned ?
regards