I did an update of my system (Arch Linux in this case) and after connecting the USB cable there appears no network interface to connect to the MOD Duo.
The solution is to restart your computer once. Hibernation or just logging out will not help. After that the interface appears (hackers can watch this running $ sudo udevadm monitor), gets an IP address ($ ip addr show) and I can connect to the GUI as normal by pointing my browser to http://192.168.51.1.
Cheers
Jakob