[SR-Users] Unresolvable destination error 478

[info at vintageelectronics.ca]

The server is installed on a Linux machine on my LAN and there are two 
users with Jitsi for a client.
One user is in the same box as kamailio (IP 10.10.10.3), another is on a 
Windows machine (IP 10.10.10.20).

Both users can connect and come online using TCP or UDP protocol.
I can see them in the output from kamctl online, but Jitsi is still 
showing them as offline.
When they try to send text messages to one another, they get below error:

The above message could not be delivered
A network problem occurred. Please check your network configuration and 
try again. Error was: 478 Unresolvable destination (478/SL)

What is the root cause and what should be done to enable them to 
communicate?

Thank you!