Azzy, seems that you're definitely right.
Below what is reported in the main page of the Minicore libs on GitHub:
"Note that you need to select Bootloader: "No" and actually hit Burn bootloader in order to set the correct fuse bits.
Now you're no longer able to upload using serial, until you select Bootloader: "Yes" and burn the bootloader again."
Shame on me.... :-[
Seems that all I have to do is:
Step 1: press "Burn Bootloader" to set the fuse bits.
Step 2: upload the sketch as before.
Now I cannot access to my little "laboratory" to test this solution. I have to wait until tomorrow, I will report my results here.
I really hope that this is the reason of the problem. I cross my fingers!