Well.... the last few days working on my GBZ project have been pretty frustrating.
I got all the components and I'm feeling like I could finish the project within one day (only need to solder everything together and map GPIOs to the buttons).
BUT there's a big showstopper at the moment:

When everything is connected in my scratch setup on the workbench, no usb device is recognized by the pi zero.
I have to mention that I had desoldered the micro USB socket and solder the wires to the test points on the backside of the board. I tried retropie and the latest release of raspbian, but it doesn't work


If I plugin a keyboard to the USB port, the LED is dark (looks like it doesn't get any power). So I took another USB hub appart and soldered this one to the pi zero, but the behaviour is still the same.
So my next step was to measure the testpoints, and I noticed that one of them was shortend to ground. Maybe I was to rough when I removed the socket from the board.
Luckily I own another pi zero, so I restarted the whole project from the beginning, in this case I haven't done anything on the sockets, I've only soldered my USB hub directly on the ports and powered on the pi, but the problem is still the same. No USB deveice is working. The Keyboad is working fine on my laptop.
So before I buy the third USB hub start my thir unsuccessful attempt, I want to ask you guys, whether someone else has another Idea, what the reason for my problem could be?
Or do you have any idea what I could test/try in addition?
I have to mention that I changed the USB hub, the raspberry pi, and also the wires from pi to the usb hub. Measuring the voltage of the USB ports, I see that they got the neccessary 5 Volts (~4.9V).
tl:dr: two different usb hubs are not working on two different rapsberry pis