Ruh Roh. Does Rivendell respond to buttons? If so it means the link is up to the tower.
One other thing you can check is whether the sources are present on any of the router selectors or UI autofill on other nodes which would indicate whether the multicast is still running.
The only other remote debugging (other than a reboot of the Rivendell server) would be log into the switch and check the ports/ARP tables for the corresponding MAC address. Though if it’s still the SG500 I installed in 2015 the web UI was not working properly on the tower unit.
—Nick