This failure was observed once in jdk 9 b115 promotion testing. It looks to be rare and no evidence to be related with test environment. I re-ran with the same build and OS for 100 times but no fortune.

Test output:
----------messages:(3/183)----------
command: main -Djava.net.HttpClient.log=all QuickResponses
reason: User specified action: run main/othervm -Djava.net.HttpClient.log=all QuickResponses
elapsed time (seconds): 0.651
----------System.out:(0/0)----------
----------System.err:(16/1010)----------
java.util.ConcurrentModificationException
at java.util.LinkedList$ListItr.checkForComodification(java.base@9-ea/LinkedList.java:970)
at java.util.LinkedList$ListItr.next(java.base@9-ea/LinkedList.java:892)
at Server.activity(Server.java:54)
at QuickResponses.main(QuickResponses.java:79)
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(java.base@9-ea/Native Method)
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(java.base@9-ea/NativeMethodAccessorImpl.java:62)
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(java.base@9-ea/DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(java.base@9-ea/Method.java:531)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:110)
at java.lang.Thread.run(java.base@9-ea/Thread.java:804)