Hello @erania-pinnera, thanks for your suggestion.
I do appreciate your intention to help.
Unfortunately, also following closely your indication (deleting plugin via FTP and then reinstalling it), once connecting Jetpack to my account, I receive the same message: “Invalid request…”
This is weird, since my other wordpress installation under the same provider (altervista.org) works perfectly fine with Jetpack.
Don’t know exactly how can I solve the issue…
Marco