It worked. The "trick" is very simple and consists in to set the passphrase (pp) IQUAL to Opera user password. Yes, you MUST set a pp if you want to sync passwords and avoid to sign in every time you use the app.
Basically, I managed to sync three different devices by following next steps:
Activating "synchronize passwords" in Opera Browser (Windows).
Will be prompted to set the pp. It is Mandatory be equal to user password.
To sign in at Opera (Android) and to activate "sync passwords".
Will be prompted to write the pp set before in second step.
Repeat 3rd and 4th steps for every Android device.
I had read your spanish text reply that was originally the solution and it was deleted by the moderator. Better the language instead the solution?