loginctl: Failed to look up user $USER: No such process

When you get this error message from e.g. loginctl show-user [...] after you deleted a user from your system:

# $USER is the username you deleted
loginctl: Failed to look up user $USER: No such process

then you can either switch to a Linux distribution without systemd or restart systemd-logind to fix this problem.

systemctl restart systemd-logind