Known issue not only in LineageOS. The problem is somehow caused by the new, improved camera modules.
And before you ask … that the phone reboots when it is supposed to shut down is caused by the new camera modules, too 
.
- Turn off the phone (reboot to recovery or Fastboot mode and remove the battery).
- Disassemble the main camera module.
- Boot the phone without the camera and encrypt, this should work now.
- Turn off the phone.
- Reassemble the main camera module.
- Boot the phone.
And … after successful encryption the phone will shut down if you shut it down at the point where it asks for the encryption password when it boots .
Edit: If it still wouldn’t encrypt (can happen after a prior failed encryption attempt) …