I had the same issue with an update to 3.71.0 of a brand new HELIX unit. The message "Updating UI MCU Firmware, please wait. Do not power off the unit!" was always showing up, no matter with footswitches was pressed at startup. After excluding a problem on my Mac, Line 6 Updater, configuration and USB cable by successfully connecting another unit, the service center advised my to contact the store that did sell me the device as this must be an hardware issue. The store MusiX was very supportive and by intervention of the manager they will give me a brand new replacement device in the next days (no available at the moment as it is a platinum limited edition). The Line 6 support is arguing that the update procedure is robust and that, should be a failure occur, the the device can be always safely restarted and the update performed with Line 6 updater by starting the unit having footswitches 6+12 pressed at startup. If this is not working then, according to Line 6 support, it must be an hardware issue. HOWEVER, as I am a computer engineer and known what you can do badly in programming an update procedure, I will be prudent in the future and eliminate any situation which could interfere with the update process: (1) deactivate screen saver (2) deactivate sleep mode (3) stop all running application (4) start and keep the updating software, either HX Edit, or better Line 6 Updater fullscreen and always in focus..