Hello Florian.
Sorry for confusion, Full Device Control functionality is not available for FREE users.
Based on error that you got, something wrong with ADB drivers.
Please try to delete ADB drivers and Device Gateway from your ZAPFARM server. Then please go ZAPFARM help > Plug in Devices and follow instructions for Android devices (See attached screenshot).
Please note, when you use Android device first time with ZAPFARM and USB debugging, you probably will see some confirmation message on your device. Please make sure you click approve.
If issue still appears, please send Gateway Log and Device Log.
Thanks,
ZAPTEST Team