USB keyboard misbehaves in 2.6.1

I have a usb keyboard wich happens to also have a standard ps2 connector (A microsft internet keyboard pro).
After booting in 2.6.1 one key (the one with ] and } on it) behaves like print screen when I only connect it via usb, when I connect it to the keyboard port as well it behaves correctly and prints the expected characters.
My keyboard has a br-abnt2 layout, but I ignore this fact and load no keymaps, since I prefer the us layout for historical reasons and it should print \ and | as it does when not working as a usb keyboard.

This key generates keycode 84 in usb mode and 43 in standard mode.

Is there any other info that could be usefull?