I found the following different behaviour for DeskTop and Device. When I in the coding, executed because of an event on Form2, sets the focus on a control on Form1:
I only wanted to share this experience with others. The solution is simple, as soon as you know the cause. It took me quite some time to find the cause in a, for me, rather complicated program, because that branche of the program was only used in case of user input errors.