tl;dr:
- Remove the new 12mp/5mp camera modules
- Reboot the phone without these modules
- Perform the encryption
- When everything is completed, put the new modules back in place and reboot the phone
Previous answer:
Apparently you cannot encrypt the FP2 if the new 12mp/5mp front and back camera modules are installed.
I had put in the new 12mp/5mp modules in before I started all the software setup of my new phone. After putting the legacy 8mp/3mp camera modules back in, the encryption went through without any hickup.
Switching to the new modules after completing the device encryption worked as well without problems.