Today I received the alert about the new FP2 system update to version 21.12.0-rel.1, and tried to install it using the update app. As has happened a few times to me before, after showing the turning circle the install failed with the belly-up Android image and the message “error.”
This time I tried a few possible extreme solutions and found one that solved the problem. The solution is to restart the phone on recovery mode and wipe the cache. After restarting the phone and proceeding with the update, it showed the turning circle and went all the way to the end of the update smoothly: problem solved.