Black Post Process Material Thumbnails/Previews

Hello,

I just started learning UE4 and made a couple of Post Process Materials.
However the thumbnails in the Content Browser turn full black, whenever I use SceneTexture:Whatever in them.
The preview in the Material Editor is black, too.

Is there a workaround or a Best Practice to get any thumbnail for post process materials? For example I noticed that Material Functions generate nice thumbnails, when all inputs have good default values.

I already tried the Thumbnail Edit Mode and the thumbnail/Preview settings in the material editor details panel.

Thanks