For the people who encounter the same problem or just stumble on this post by coincidence:
UniPi tech wrote: “We know the firmware update, especially of the extensions, is a sort of Achilles heel and we are thinking about some concept of dual firmware to which you can revert to if the flashing of the new firmware fails.”
I also would like to mention the excellent support from UniPi! I’ve received a very fast and meaningful response from both the technical as commercial team, and they are moving heaven and earth to provide me the fastest possible solution.
I didn’t experience such a personal approach from an international company since a long time.