A few days ago I've received as a gift a nice ATS-Decoder Mega with fw version "4.16 Air" and a (I suppose and hope) regular activation key, all is working fine and I'm quite happy with it

Now the problem I've had: I've discovered the existence of a more recent FW version 4.20, that has a lot of very nice new features, and I tried to update it. I'm not sure if I still have to purchase a new activation key, or if the current one will work as well, but this isn't the issue yet, since the update procedure has always failed. This is what I've done so far:
- I've downloaded the .exe firmware installer for 4.20, and I've asked a friend to use his spare PC for the task (I do not have any Window PC, being a Mac user), it has Windows 7 Home Premium OS.
- I've installed the serial drivers (no problem)
- I then tried multiple times to update the firmware, but without success. This is diagnostic:
The radio connects without issues (the software finds the right COM port), when I click fw update it start to transfer files to the radio, and the display of the radio shows a bow with "Downloading files ..." for a couple of times, then a sort of "terminal" windows appears on the PC screen, for a minuscule fraction of a second (I cannot read what it says, but looks like a DOS/unix terminal windows, mostly black), and the upload process stops. The software writes this text on the on-screen-log:
-> Save data from device:
value.hex
ap.csv
decoder.log
...skipped
decoder.idx
...skipped
lang.tmp
...skipped
...disconnected
-> Install firmare...
...failed
After this, the radio is still working fine, with the old fw.
Anyone has an idea of what is happening and how I can solve the issue?
Thank you very much in advance,
Carlo (Italy).