Changing nameserver allowed both of my broken Piholes to update FTL, and resolved my problem: Thanks deHakkelaar!
Error: Unable to get latest release location from GitHub
connection timed out; no servers could be reached
Temporarily change nameserver in /etc/resolv.conf from 127.0.0.1 into 8.8.8.8 :
sudo sed -i 's/127.0.0.1/8.8.8.8/' /etc/resolv.conf
And run repair:
pihole -r