2017.12.08 21:29:29 ERROR [pool-4-thread-1]: org.jivesoftware.openfire.container.PluginMonitor - An unexpected exception occurred:
java.util.concurrent.ExecutionException: java.lang.NullPointerException: Plugin could not be located: clustering
at java.util.concurrent.FutureTask.report(FutureTask.java:122)
at java.util.concurrent.FutureTask.get(FutureTask.java:192)
at org.jivesoftware.openfire.container.PluginMonitor$MonitorTask.run(PluginMonitor.java:310)
Is this actually a cluster? Did you download updated plugins or are you running updated plugins in an older openfire?