Are you connecting via UDP to the serial ports on the PortServer unit or is the PortServer unit establishing a UDP connection to a server on the network?
At any rate, for UDP server configuration (i.e. incoming connections to the Digi serial port), the default port is listed when configuring the UDP Socket profile on the serial port, where 2101 = port 1, 2102 = port 2, etc.... You cannot have multiple connections open on a single port simultaneously unless you have the port sharing feature enabled (Configuration > Serial Ports > Advanced Serial Settings > Allow multiple systems to simultaneously connect).
For UDP Client configurations, you can provide the IP address and port to send the serial data to. It does not appear to complain about duplicates here (although there should not be any reason to list the same twice).
For testing, you will need something that will establish an UDP connection, such as netcat.