Problem with Wii U Pro Controller on Steam

[manuel]

I'm using a wireless adapter from 8BitDo to connect a Wii U Pro controller to my Windows 10 computer.

So far so good.

In the  Win 10 controller configuration/test (joy.cpl) all buttons and sticks work without a problem.

Same on gamepad-tester.com

Everything works.

But when I try to use it on a steam game, the directions left/right on both sticks don't register at all. Up/down works, but not left/right.

Has anybody ever had a similar problem and maybe a fix?

It's a perfectly working and good controller, but this is driving me mad, because I don't understand why left/right won't work.

(I already tried re-installing the device/driver, no luck.)

Thanks in advance!