After many tries this afternoon it connected, but wouldn't send messages; after trying again it hasn't connected at all. It says connection failed, locks up on "connecting" or presents me the registration menu.
Re-registration doesn't work because the WhatsApp service still has your phone registered, it's the client itself that can't connect - so official WhatsApp under NitDroid will work just fine.

I think something has changed in the server, not sure what.

Scorpius, if you for whatever reason are unable to fix it would you mind opensourcing Yappari? It'd be a great help for those of us who still want to use their N900.