On geoserver I have two layers, both cascading a remote WMS. Both layers work fine in the layer preview.

Now I am putting both into one group layer and open the preview, which shows an empty response. When trying to query the group layer in the browser with

> localhost:8080/geoserver/ws/wms?service=WMS&version=1.1.1&request=GetMap&layers=ws:layername&styles=&bbox=6.03777,48.898,14.703518328104952,54.93599775300893&width=768&height=535&srs=EPSG:4326&format=image/png

, I get the following response:

> Rendering process failed Argument "bgColor" should not be
> null.

When I add `&bgColor=0xFFFFFF` to the request, I still get the same error response.

The same happens, when I call the underlying layers within the `&layers` parameter, without using the group layer.

I thought, it might be about different wms implementations on the remote WMS side, but even when I switch to different remote wms services, the problem persists.
When googling, I didn't find a response that seems to solve the problem. Do I miss something?