Okay, I succeeded! I just skipped the zip signature verification. Thanks a lot for your patience!
The OS looks quite unstable, though. For instance, when I select WiFi settings, Bluetooth settings open. Or, if I select a wifi provider, a different one is shown. Windows appear double or in multiple quantities covering the whole screen, and in some cases keep moving up and down. Have you experienced anything similar?
Do you know how to take a screenshot? If it works, I can show examples of what I mean...
Is there something I need to do at the end of the flashing, like locking the device again? Shall I use fastboot for that?