Resolution:
There is another process using the default port that RTserver uses (5101).

An example of an application the uses port 5101 would be Yahoo Messenger. The Yahoo Messenger connects to the yahoo server(cs.yahoo.com) on port 5050/TCP for client-to-server communications . It then listens for peer-to-peer requests on port 5101/TCP which is the default port used by RTserver upon startup.

$ netstat

Will show Yahoo Messenger at Port 5050

$ netstat -a

Will show Yahoo Messenger LISTENING at Port 5101

The IANA (Internet Assigned Numbers Authority) shows the following ports as registered to Talarian Corporation, now owned by Tibco.
From http://www.iana.org/assignments/port-numbers

talarian-tcp    5101/tcp   Talarian_TCP
talarian-udp    5101/udp   Talarian_UDP

You can change the port number RTserver uses with the RTserver setopt conn_names option:

setopt conn_names tcp:_node:5799 // unassigned by IANA

Then the RTclient applications would set their server_names option as follows:

setopt server_names tcp:_node:5799  // or replace "_node" with the name of the machine where RTserver is running

Another alternative is to start RTserver before starting Yahoo Messenger, then RTserver can use the default 5101 port, and Yahoo Messenger will automatically listen on a different port.