it's me again.
I guess I am a bit closer. I was wondering why my custom gamepad driver which generates a uinput device is not working. By incident I started the game directly without the start script, so I could see the information output. One of the very first lines was "couldn't find keyboard event file". I recognized that my custom driver creates a eventX in /dev/input but not in the subdirectories by-id or by-path.
So I tried to create a symbolic link in by-id, pointing at my event.
Now the keyboard-emulating gamepad worked in the game!
Playing that a bit further I went back to my gamepad driver which behaves like a gamepad without keyboard emulation. Running on top xboxdrv with mouse and keyboard emulation. again with a symlink in by-id.
Now the regular gamepad-driver with xboxdrv on top worked in the game!
Only point is, that no regular keyboard is allow to be connected (maybe be game only uses the first keyboard?).