I have uninstall and reinstall the Monitoring plugin.
I also got the following error:
org.jivesoftware.openfire.container.pluginServlet - java.lang.NullPointerException
at org.jivesoftware.openfire.reporting.graph.GraphEngine.populateData (GraphEngine.java:182)
at org.jivesofteware.openfire.reporting.graph.GraphEngine.generateSparklineareaCha rt (GraphEngin.jave: 445)
at org.jivesoftware.openfire.reporting.graph.GraphEngine.gneerateSparklinesGraph (GraphEngine.java: 169)
at org.jivesoftware.openfire.reporting.graph.GraphServlet.service (GraphServlet.java: 125)
at javax.servlet.heep.HttpServlet.service (HttpServlet.java:820)
...