Still, when I start Yappari, I get "Connecting", and after couple seconds "Connection closed by server." It tries to connect repeatedly, but fails.

I upgraded to the latest Yappari 2.0.10 Build 5210
With the older version I got blocked, at least that was the error message when I tried to enter my phone number. I was able to lend an old Samsung from a friend, upgrade the official client, and send a message to Whatsapp, asking them to unblock me. Maybe there was something wrong with the message I sent, as it seems I was never unblocked.

Should I have held on to my friends old phone, trying to make the connection with that first, and only when I got that connected, switch to N900? Now I just sent the message, gave the phone back to my friend, waited couple days and tried with N900.