finally my wii u pro controller arrived in the mail today. first i tried it to pair without any dongle in recalbox 4.0. as expected nothing.
then i plugged in a random dongle i found in one of my drawers. plugged that baby in one of the usb ports, booted up recalbox, went to the menu to pair it. and IT WORKED. no hassle, no nothing. it works 100% as far as i can tell. i managed to configure every button and it works in menu as well as in-game.
so i don't think you need to fix anything but to enable the internal bluetooth.
for everyone who is interested in what bluetooth dongle i used: "logilink bt0013".
thank all of you again for your time.
happy gaming!
cheers
tim