I got now an answer from the Qlik Support.
They have and hard coded limit of max. 50 MB what can be transfers from the master app to the template app via HTTP.
So all request with more objects than that limit will run into an error: "Payload too much".
That means for me the On-demand-app-generation is a nice gimmick but for bigger data not usable
Also I don't think HTTP is right protocol/technique for this purpose, if I already have all my data in memory....