Thank you very much for your answer.
Although there was a slight misunderstanding in my issue. The margin issue is not inside the RadRibbonWindow, it's one OUTSIDE. That means, if I maximize the window, my whole screen is not covered. There is a slight margin on the right side, where you can see through to the desktop. Therefore in my first screenshot, the black color is my desktop background color.
In the 2014 version, the whole Screen was covered like it should. In the 2015 version, the Screen is only partial covered and there is a slight margin on the right, where you can see through to the Desktop.
I hope I could make the issue clear.