@bjorn,
I am not seeing the same 404 as you. See attachment #1.
@rscheitlin,
Yes, adding the layers query param to the URL will export the image. The problem is that the API is not adding that query param to the URL. See attachment #2. How did you get that URL above? Did you manually add the &layers=show%3A1%2C2 part, or is that how the Flex API constructed the URL?
Thanks,
Mike