Hello Christian, thanks for the post.
The VGS has software libraries and support for both the Wii Nunchuck and the Classic Controller, including both official Wii controllers and knockoff controllers (at least the knockoffs we have in our store).
Due to the differences between the nunchuck and classic controller, there is a separate library for each. Try running the classic_controller_demo example in the ClassicController library, that should give you more debug information about what the controller is reporting for button presses, joystick positions, etc.
Hope that helps, let us know what you find!