Problem

When I wanted to upgrade my ArchLinux installation I got an error about a failed lookup for a key:

error: key "<KEY ID>" could not be looked up remotely
error: required key missing from keyring
error: failed to commit transaction (unexpected error)

Solution

Turned out that reinstalling archlinux-keyring loaded the key:

sudo pacman -S archlinux-keyring
sudo pacman -Syu