Why I can´t see any Material Insatance in UE 5.1 anymore?

Till several day´s I got a problem to see any material instance in the viewer. I see only a black background. How can I fix this?

I´m on a Ryzen 9 5900, 64 GB RAM, Nvidia 3090Ti, Win 11.

In the main editor window:

Window → Load layout → Default editor layout