@danfro I do not fully remind the details but what I did was:
1- Erase all the app data on the UT Cinny (using UT Tweak Tool)
2- Remove the whatsapp bridge from Beeper (I had it installed on another phone to confirm the bridge was working, set up the configuration, etc). Not sure how I did but, if I remember well, there is an option on the network to remove whatsapp.
3- Reconfigured the Cinny on UT (i.e. to follow the steps to allow the encription topic)
4- To build the bridge with Whatsapp again on Beeper on the non UT phone
5- Open Cinny on UT phone.
That decrypted the messages on Cinny.
Later on, although I guess not mandatory nor needed, I removed Beeper installation from the non UT phone.
I hope it helps.