@Substring @lackyluuk
Had few time the last week, but finally managed to try it this way. Didn't work.
When I run the commands and restart Recalbox, the Joystick doesn't work and as soon as I hit one of the buttons, a window pops up: "configure input. 2 Gamepads detected". As soon as I try to configure, I get the same problem as befor: everything works fine, I can configure the arrow keys, abxy and start, as soon as i get to the select-button. Can't configure select or any of the following buttons.
So I finally gave up and completely reinstalled Recalbox. Didn't help. Same issues.
In my desperation I just hit any buttons while configuring and found out something: the select button (the one I want to be the select button in my build) works fine when I try to configure it as any other button. As well, the other buttons, that didn't work befor, can be configured as any of the arrow, abxy or the start button. But again, as soon as it gets to configure the select button in recalbox, nothing works.
So finally I guess, the problem isn't any of the hardware-buttons, but recalbox just lets me configure up to 9 buttons and not 12 buttons, as it should be.
Sorry for my pour english, I hope I can make you understand, what I mean.