First check: MD5 checksum. It was correct.
Second check: fastboot mode.
a) with “Volume up” key pressed: led turns to green and then to blue; fastboot recognizes the device. I do not know if it is important or not, adb does not recognize the device
b) with “Volume down” key pressed: led turns to green, but neither fastboot nor adb recognize the device
Now, I am stuck at the step “Then try to flash /e/ again”: I guess it means I should repeat all the steps of the installation procedure from the paragraph “Temporarily booting a custom recovery” on, but as adb does not see the device, how can I proceed? I do not understand why adb does not see the phone and fastboot does, but I guess the good question is: how can I make adb see it?
Thank you again for your support and patience…