After intense research, no answers and hours of experimenting, I can assume that the physics from gamestudio are not able to let a object jump properly(!).

Or maybe: Im not able to let a object jump properly. So please, let's fill this ridiculous knowledge gap

PS: And no, I can't try to use newton, because I can't update my A7-version, because I can't download the update, because someone has ruined my adsl and downloading 1 mb needs 2 hours (and costs 2 €)