How to change Z39.50 port in Geonetwork fixing the error "Address already in use"?

Question

I have deployed two portals (on two tomcat instances) each of which are based on Geonetwork 2.6.0.0 which use Z39.50 protocol. By default the port for both of them is 2100 which causes the second portal to give the following error:

ERROR [org.jzkit.z3950.server.Z3950Listener] - Problem
java.net.BindException: Address already in use
    at java.net.PlainSocketImpl.socketBind(Native Method)
    at java.net.AbstractPlainSocketImpl.bind(AbstractPlainSocketImpl.java:376)
    at java.net.ServerSocket.bind(ServerSocket.java:376)
    at java.net.ServerSocket.<init>(ServerSocket.java:237)
    at java.net.ServerSocket.<init>(ServerSocket.java:128)
    at org.jzkit.z3950.server.Z3950Listener.run(Z3950Listener.java:69)

I tried to change the port according the instructions given in the following URL: https://geonetwork-opensource.org/manuals/2.10.4/eng/developer/xml_services/system_configuration.html To change the port, in the portal, I used xml.config.set service which refers to the Java class: org.fao.geonet.services.config.set But when I sent the information containing the new port using http POST to the xml.config.set, the response was not a success:

Error   org.jdom.text cannot be cast to org.jdom.element

Both portals are using jdom-1.0.jar. I tried newer versions of jdom. But I didn't succeed to give two different Z39.50 ports to the portals.