Thanks a lot for your support. I have changed the following in order to get it working for now:
Note, You may want to disable the various mux rewriting options in Configuration -> Stream -> Stream Profiles -> Pass, "Rewrite *** etc..
and changed calling the channels in the python script to
http://ip:9981/stream/channelnumber/1
Now at least my first transcoding works. Thanks!
Well, does anybody know how I analyze (maybe via log file) in which stream configuration a channel will be provided (e.g. raw ts)?