Thanks for the useful hint, indeed pressing the BLUE key while turning on the GD-77 does the trick!
Before doing this the option "Mode: Auto" was missing, there were only "Mode: None" and "Mode: Manual".
To re-enable the missing option, I also had to act randomly even by turning the transceiver off and on again.
In detail, what I noticed is the following:
By pushing the GREEN button I entered the menu and from there I selected the "Display option" item where I chose "Manual".
Once this was done, the backlight was controlled by the side button SK1 (very convenient option!).
Wanting to restore automatic operation, I did the same steps as before but the possible choices were only "Mode: None" and "Mode: Manual", from which always "Timeout: N/A", until by acting randomly on the GD-77 it was shown to me the missing option "Mode: Auto".
Now after pressing the BLUE key and turn on the transceiver the problem has gone away and everything works as expected.
Probably this thing about turning on the GD-77 with the blue button pressed after an update is supposed to be mandatory, but I had missed it, my fault, sorry.
From now on I will not fail to do it every time, thank you Daniel!
73 best regards de Fabio IZ2EIB