X_TRANSPORT_UNAVAILABLE

the fanout bus is unreachable

the nats-server NATS_URL names is down or misconfigured. At boot only a sync node or a replicator raises it, after retrying the dial for 15 s — the cause names the server, the wait and the last attempt's own failure. A web, worker or scheduler process boots without the bus and raises it from a publish instead (publishChannelEvent, the cache-invalidation broadcast), at once, until its background dial lands; a publish made once the client's socket has closed is refused the same way rather than buffered by the client; the replicator retries such a refusal three times over 2 s before its run ends (Realtime → what each role needs from the bus)

x doctor — then check NATS_URL; in app code, catch it around a publish and log — the event meant "re-read"