Thanks for all the help,
but as so often, the error lies between chair and keyboard :-/
Yes, all my observations were seemingly all correct, but for all the wrong reasons. The problem was simple: My message class had no parameterless constructor and only private setters. It seems, that while the XML-Serializer doesn't have a problem with this, the JSON serializer does.
After making the properties get / setters public, I finally get my message returned. Bummer.
Especially a big thank you towards Particular, who made a big effort in trying to help me. I guess, all I needed were a few days off =)