I'm in the process of getting a Wii U, so I'll be sure to look into it within the next few weeks. But it'd be good to know if something like r1272 (mod v7) has the same problem.
I assume you're using Nintendont 6.500 and that everything works correctly when you don't use the forwarder to have gamepad support?
I am pretty sure that is a long standing bug that even had some input by Fix94 back when cyan first introduced WiiVC Inject compatibility with the WiiU gamepad. I had to disable args boot to avoid a black screen. If I recall correctly this occurred when usb hubs or HID devices were connected. @Kirby567fan do you have any non-direct usb devices connected?
edit: blackb0x let me know if you would find value in me pulling together some old threads/comments of testing and discussions related to this. I haven't gotten a chance to try your new release (congrats btw) because my WiiU is still configured with a RedNAND partition on the SD.