I managed to "switch" unencrypted messages back on! I had the same issues after using the android version of whatsapp lately. Luckily I can call an ol' Nokia E7 my own but I guess any Symbianphone will do the trick. (Or maybe there is an Emulator/ VM image out there?)

1. Uninstall WhatsApp Android from your jolla! (Don't dare to use an official and an unofficial version of "whatsapp" simultanously)
2. Power off your Jolla and switch your SIM to an Symbianphone (e.g. E7)
3. It's likely that you have to set up Date and Time on the Symbiandevice (Your Time and Date has to be set, otherwise WhatsApp won't start)
4. It's also likely that you have to update WhatsApp on the symbiandevice. If WhatsApp refuses to start do so.
5. Register to WhatsApp from your Symbian Device.
6. Write some Massages to anyone (I don't know if it's important, I did so.)
7. Turn off the symbiandevice and switch the SIM back in your Jolla
8. In case whatsup 0.1-10 is installed, open it. Otherwise install what the version provided by the forrestcats (AGAIN MAKE SURE OFFICIAL WHATSAPP CLIENT IS UNINSTALLED OR EVEN ANDROID IS STOPPED)
9. Register to WhatsApp using whatsup (If still installed wait for the connection time-out)
10. If whatsup tells you to wait for xx min, DO SO. It's getting worse everytime you don't.
11. Voilà, everything should work properly.

I tested sending messages to my Jolla using whatsup from a Galaxy S4 with official WhatsApp client and everything worked.

Since this is my first post here I'd like to thank everyone who gives time and effort in this and simialar projects. You guys rock!

EDIT: If there is an Symbian Emulator or VirtualMachine Image available (no idea) you have to keep your Jolla With SIM inserted on (obviously for registration) I recommend to stopp Alien Dalvik, so any WhatsApp service is killed.