I think this error is happen when you use UE4.27.2 and 5.0.2
you need to set your IP address manually for unicast endpoints
first launch UE from epic games launcher.
and open ndisplay BP Editor → ndisplay node’s IP address and change it with your actual IP address not (127.0.0.1)
(as i remember Epic’s past official document said it’s good to use 127.0.0.1 but i guess they update. it’s not working with 127.0.0.1)
then you need to go to Switchboard’s setting and add your actual IP address espacially
Switchboard’s settings → nDisplay settings → Unicast Endpoint → add your IP address(example 123.123.12.123:0) do not delet port number
Switchboard’s settings → nDisplay settings → node_0 → IP address → use your actual IP address
Switchboard’s settings → Unreal settings → Unicast Endpoint → Add your IP Address with port number :0
Editor’s IP address too
and don’t touch switchboard setting’s multi user server unicast endpoint