That’s a message when the relay (or server) is attempting to notify child relays about new actions or site updates. It implies there is a downstream relay that is unreachable or offline.
The downstream RelayNotifer traffic uses TCP/52311, as opposed to the client notifications that use UDP. Check whether TCP downstream traffic is blocked.
If there is a NAT or other rule that cannot be overcome, it’s safe to ignore the message, but you might want to implement Command Polling so that child relay and it’s clients respond quickly to actions; and BFQuery may not work when you target that child relay or it’s downstream clients.