kurz meine Lösung:
- reset button = not responsive (30++sec)
- pulled the plug
- pluged-in again while pushing reset
- conscutive messages on display: "Starting ..." >>> "will take longer" >>> "something wrong ...."
- pulled plug / plugged in again
- Restart >>> "updating USW"
- All devices showing up on UniFi Network / Devices again
- manually starting update from UniFi Network (Update-Link)
- Update OK
- admin