As Anssi implied, your graphics card may not be up to the task of handling SketchUp 2017’s tighter requirements for graphics. For a long time the Intel HD GPUs have been known to be lacking in their support of OpenGL. This is more critical now than it has been with previous versions of SketchUp. As painful as it is, it was inevitable that things would go this way.
Maybe it is time for a shiny new computer with a nice nVidia GPU. 
Yep! I’ll buy it for myself as a Xmas gift! 
SU 2017 still runs on integrated intel HD/Iris GPUs… a recent driver version implied.
@ChrisDizon No, for me everything looks ok.
I’ve also tried to install new driver as sketch3d_de sugessted, but get an error message - so I really assume that my grafics are the problem.
Regards, Dani
To rid yourself of the yellow triangle you need to render the viewport. Right click somewhere on the Layout page and select RENDER MODEL. Any time you make a change to the view port, layout wants to re-render it. In the Sketchup Model dialog window there is a check box with AUTO next to it. Checking it will automatically re-render for you.
Right click on a blank space and select render model from the context menu