Finally I tracked the problem down.
The web server has a problem with the header:
The Nokia debugger does NOT show all http-headers. I captured the http-requests of my simulator with the help of Microsoft Network Monitor.
Content-Type: image/gif; q=0.5
I wrote a ruby script to send requests myself. When I add the Content-Type Header, the server responds with 500. Without it, everything is ok.
So there are two points
1) The server is to blame, it has an error, it should not answer 500.
2) But the Nokia Tool Chain seem to have a bug also. To me, there seem to be no reason to send this header.
The header gets sended when I do a normal external static image, like this
The image is jpg, not gif!
Also, the content-type header in the request is to define body of the request. But there should be no body in this request, as I don't want to send an image, I want to receive an image. And the "q=0.5" makes only sense for the accept-header.
I have investigated this problem in NokiaWDE 1.5 WebSDKSimulator. The error also happens in the cloud preview. But I can not do a network analysis there, but I expect it is same as the error is the same.
Does anybody knows who to report bugs to Nokia nowadays?