Communication hangs when 3rd person enters conversation

OS

Ubuntu 16.04

Jitsi-meet package version:

2.0.4548-1

Problem :

Everything works fine with two people on the conversation. However when a third person joins everything hangs, i.e no video , no sound until someone disconnects. The error in jvb.log is:

java.lang.NullPointerException
        at org.ice4j.socket.MergingDatagramSocket.initializeActive(MergingDatagramSocket.java:577)
        at org.ice4j.ice.ComponentSocket.propertyChange(ComponentSocket.java:174)
        at org.ice4j.ice.IceMediaStream.firePairPropertyChange(IceMediaStream.java:870)
        at org.ice4j.ice.CandidatePair.nominate(CandidatePair.java:629)
        at org.ice4j.ice.Agent.nominate(Agent.java:1847)
        at org.ice4j.ice.DefaultNominator.strategyNominateFirstValid(DefaultNominator.java:144)
        at org.ice4j.ice.DefaultNominator.propertyChange(DefaultNominator.java:120)
        at org.ice4j.ice.IceMediaStream.firePairPropertyChange(IceMediaStream.java:870)
        at org.ice4j.ice.CandidatePair.validate(CandidatePair.java:667)
        at org.ice4j.ice.IceMediaStream.addToValidList(IceMediaStream.java:668)
        at org.ice4j.ice.Agent.validatePair(Agent.java:1811)
        at org.ice4j.ice.ConnectivityCheckClient.processSuccessResponse(ConnectivityCheckClient.java:638)
        at org.ice4j.ice.ConnectivityCheckClient.processResponse(ConnectivityCheckClient.java:405)
        at org.ice4j.stack.StunClientTransaction.handleResponse(StunClientTransaction.java:314)
        at org.ice4j.stack.StunStack.handleMessageEvent(StunStack.java:1040)
        at org.ice4j.stack.MessageProcessingTask.run(MessageProcessingTask.java:196)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
        at java.lang.Thread.run(Thread.java:748)

Any ideas how I could troubleshoot this ?
Thank you in advance

Someone please ?