Pihole update failed on Ubuntu 18.04 LTS due to "Unsupported OS"

It is likely a old (?) Microsoft server which doesn' t handle DNS cookies right. See Pihole won't install (no errors, stops after dependencies) - #21 by DanSchaper and https://www.kevinwlocke.com/bits/2017/01/20/formerr-from-microsoft-dns-server-for-dig/.

Set another upstream server and the update will likely run without issues.