The two screen shots are identical. I do see a title of "Gastsuche" on the window. As far as Test Studio is concerned window title = window caption.
If the test script is trying to click on the button before it exists, you can add a line of code to wait for it to exist before clicking on it like this:
Use this in place of the Thread.Sleep(2000);
P.S. Public forum posts have a guaranteed response time of 72 business hours, not 24 business hours. We do not work weekends, thus we do not count weekends.