Hey, it took me quite a while to find out because I had only standard game controllers and I had lost my usb adapter for my old ps2 gamepads... But with the heat coming back I suddenly thought again about this old guitar hero guitar which also has an usb adapter... that's the usb adapter I was looking for !
And these ps2 gamepads were not recognized as standard gamepads (what a surprise, my usb adapter appears as "Lakeview Research MP-8888 USB Joypad", no wonder it's not recognized ! So I mapped it using the "controller mapping" command, and button A (which is X on a ps2 gamepad actually) was not recognized in the gui !
Oh well, it's fixed finally, but it was a bug in the gui only, in game it was already perfect.
I'll add some info for the "controller mapping" command because the window appeared behind my fullscreen raine window for some reason, so if you don't know there is another window here, you can be lost !