I solved it now, but I've got no idea why this should be the solution... Just guessed it. Even now that I know the solution, I cannot find much evidence for it. Maybe I miss something?
I'm using ABAP. The only language I know well enough to get some of the challenges solved. I think it contains webclient functionality as well, but I was too ...
I'm using a programming language which does not allow HTTP sockets or similar stuff. So I thought to use HTML forms with my web browser to post the solutions manually. Works well for the challe ...