Here are some helpful troubleshooting tips for the Melvin Lee Davis Bass Preamp I got from the NUX support. They are missing in the current product manual so maybe they’re helpful for other players as well.
Problem: The editor app crashes on startup while the preamp is connected to the computer.
Solution: Reinstall the editor software. Before you connect the preamp to your computer, hold 2 foot switches, re-power the preamp again, then connect to the computer.
Problem: The preamp isn’t detected in the firmware updater although the device is detected in the OS.
Solution: You have to reconnect the preamp while it’s in DFU mode. Connect the USB cable and hold 2 foot switches and re-power to enter DFU mode.