@ KempeWeidenbaum: I don't know if this would help or if it is the correct procedure, but it always worked for me. Earlier when I was using Mitakuulluu 1, I moved to the official android Whatsapp, but was not able to able to send pictures as android didnt have access to the gallery. So I tried to switch back to Mitakuuluu, but was getting registration error, no matter what I tried. I installed android Whatsapp again, got registered and then installed Mitakuuluu and Mita was able to detect the Whatsapp account automatically.
Then upgraded to Mita ver. 2and when I recently switched to whatsup, I installed it while I still had my Mita ver 2 and I didnt have any registration problems. After my whatsup was installed and was found to be working fine, I uninstalled mita ver 2.
So, my suggestion is, have yourself registered via the official Android Whatsapp, install Cepi's whatsup and see if you are able to register / or if whatsup automatically detects the whatsapp account and if either works, then uninstall the official Whatsapp.