After I ran “ssh root@192.168.51.1 “rm /data/enable-usb-multi-gadget && reboot”” the duox disconnected again after a short time to the web gui. (Also once no “Pedalbords Libery” and “Banks” was callable).
According to instructions from:
Duo X as USB Audio input and MIDI device | EXPERIMENTAL
Then I executed “ssh root@192.168.51.1 “rm /data/enable-usb-audio-gadget && reboot”” and I was able to connect to the modx Web Gui again successfully. I’ll let you know if it’s permanent, but it’s been running very stable for a few minutes now - without any interruptions.
Just checked again: The Web Gui is disconnected again after both functions turned off. 
According to instructions from:
Audio Through USB - MOD Wiki
Are both necessary commands or only the usb-multi-gadget necessary? On the wiki both are listed, in the forum only the usb-multi-gadget.
What I noticed is that the individual pages (“Pedalbords Libery”/“Banks/…” load much faster when it is switched off. With usb-multi-gadget/usb-audio-gadget it took ~very long.