Microsoft Ribbon does not render in Designer anymore

Hello,

I'm using the Microsoft Ribbon for WPF (System.Windows.Controls.Ribbon) and VS2015. Since I removed unnecessary depencies from my project (like Dynamic Data Display etc, things not related to the ribbon) it does not render correctly in the Designer anymore. All RibbonButtons get rendered as usual buttons and so on.

I used the Clean Solution option and also rebuild multiple times, without sucess. While execution everything is fine, just the designer can't handle the ribbon anymore. What could have caused this strange behaviour?

Sadly MSDN does not allow me to include links or pictures in this post at the moment but I could send them if anyone is interested.

Greetings,
Marcel