EventCalendar Exception after creating Events

Hello,
I just tried to create some CalendarEvents.
After creating them I'll be able to see the Events.
But after some time I'll get the following exception if I want to watch a month which has some events.

Error number 4001 in 4: Error while parsing velocity page Main.EventCalendar
Wrapped Exception: Invocation of method 'getHTMLCalendar' in class com.xpn.xwiki.plugin.calendar.CalendarPluginApi threw exception java.lang.NullPointerException @ Main.EventCalendar19,8?
com.xpn.xwiki.XWikiException: Error number 4001 in 4: Error while parsing velocity page Main.EventCalendar
Wrapped Exception: Invocation of method 'getHTMLCalendar' in class com.xpn.xwiki.plugin.calendar.CalendarPluginApi threw exception java.lang.NullPointerException @ Main.EventCalendar19,8?
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:160)
at com.xpn.xwiki.render.XWikiVelocityRenderer.render(XWikiVelocityRenderer.java:95)
at com.xpn.xwiki.render.DefaultXWikiRenderingEngine.renderText(DefaultXWikiRenderingEngine.java:252)
at com.xpn.xwiki.render.DefaultXWikiRenderingEngine.renderText(DefaultXWikiRenderingEngine.java:176)
at com.xpn.xwiki.XWiki.include(XWiki.java:3350)
at com.xpn.xwiki.api.XWiki.includeForm(XWiki.java:1362)
at com.xpn.xwiki.api.XWiki.includeForm(XWiki.java:1328)
at sun.reflect.GeneratedMethodAccessor234.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:295)
at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:245)
at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:203)
at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:294)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at org.apache.velocity.runtime.directive.VelocimacroProxy.render(VelocimacroProxy.java:194)
at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:170)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:241)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:155)
at com.xpn.xwiki.render.XWikiVelocityRenderer.render(XWikiVelocityRenderer.java:95)
at com.xpn.xwiki.render.DefaultXWikiRenderingEngine.renderText(DefaultXWikiRenderingEngine.java:252)
at com.xpn.xwiki.render.DefaultXWikiRenderingEngine.renderText(DefaultXWikiRenderingEngine.java:176)
at com.xpn.xwiki.render.DefaultXWikiRenderingEngine.renderText(DefaultXWikiRenderingEngine.java:145)
at com.xpn.xwiki.render.DefaultXWikiRenderingEngine.renderDocument(DefaultXWikiRenderingEngine.java:134)
at com.xpn.xwiki.doc.XWikiDocument.getRenderedContent(XWikiDocument.java:418)
at com.xpn.xwiki.api.Document.getRenderedContent(Document.java:371)
at sun.reflect.GeneratedMethodAccessor226.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:295)
at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:245)
at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:203)
at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:294)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:241)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:155)
at com.xpn.xwiki.XWiki.parseTemplate(XWiki.java:1415)
at com.xpn.xwiki.api.XWiki.parseTemplate(XWiki.java:598)
at sun.reflect.GeneratedMethodAccessor143.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:295)
at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:245)
at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:203)
at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:294)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at org.apache.velocity.runtime.directive.VelocimacroProxy.render(VelocimacroProxy.java:194)
at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:170)
at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:74)
at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:88)
at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:74)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:107)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:241)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:155)
at com.xpn.xwiki.XWiki.parseTemplate(XWiki.java:1415)
at com.xpn.xwiki.web.Utils.parseTemplate(Utils.java:108)
at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:199)
at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431)
at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at com.xpn.xwiki.web.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:117)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:215)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:174)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:151)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:874)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:689)
at java.lang.Thread.run(Thread.java:619)

Wrapped Exception:

java.lang.NullPointerException at com.xpn.xwiki.plugin.calendar.CalendarData.getContent(CalendarData.java:242) at com.xpn.xwiki.plugin.calendar.CalendarPlugin.getHTMLCalendar(CalendarPlugin.java:192) at com.xpn.xwiki.plugin.calendar.CalendarPlugin.getHTMLCalendar(CalendarPlugin.java:95) at com.xpn.xwiki.plugin.calendar.CalendarPluginApi.getHTMLCalendar(CalendarPluginApi.java:60) at sun.reflect.GeneratedMethodAccessor236.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:295) at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:245) at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:203) at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:294) at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318) at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:241) at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:155) at com.xpn.xwiki.render.XWikiVelocityRenderer.render(XWikiVelocityRenderer.java:95) at com.xpn.xwiki.render.DefaultXWikiRenderingEngine.renderText(DefaultXWikiRenderingEngine.java:252) at com.xpn.xwiki.render.DefaultXWikiRenderingEngine.renderText(DefaultXWikiRenderingEngine.java:176) at com.xpn.xwiki.XWiki.include(XWiki.java:3350) at com.xpn.xwiki.api.XWiki.includeForm(XWiki.java:1362) at com.xpn.xwiki.api.XWiki.includeForm(XWiki.java:1328) at sun.reflect.GeneratedMethodAccessor234.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:295) at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:245) at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:203) at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:294) at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318) at org.apache.velocity.runtime.directive.VelocimacroProxy.render(VelocimacroProxy.java:194) at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:170) at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318) at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:241) at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:155) at com.xpn.xwiki.render.XWikiVelocityRenderer.render(XWikiVelocityRenderer.java:95) at com.xpn.xwiki.render.DefaultXWikiRenderingEngine.renderText(DefaultXWikiRenderingEngine.java:252) at com.xpn.xwiki.render.DefaultXWikiRenderingEngine.renderText(DefaultXWikiRenderingEngine.java:176) at com.xpn.xwiki.render.DefaultXWikiRenderingEngine.renderText(DefaultXWikiRenderingEngine.java:145) at com.xpn.xwiki.render.DefaultXWikiRenderingEngine.renderDocument(DefaultXWikiRenderingEngine.java:134) at com.xpn.xwiki.doc.XWikiDocument.getRenderedContent(XWikiDocument.java:418) at com.xpn.xwiki.api.Document.getRenderedContent(Document.java:371) at sun.reflect.GeneratedMethodAccessor226.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:295) at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:245) at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:203) at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:294) at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318) at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:241) at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:155) at com.xpn.xwiki.XWiki.parseTemplate(XWiki.java:1415) at com.xpn.xwiki.api.XWiki.parseTemplate(XWiki.java:598) at sun.reflect.GeneratedMethodAccessor143.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:295) at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:245) at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:203) at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:294) at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318) at org.apache.velocity.runtime.directive.VelocimacroProxy.render(VelocimacroProxy.java:194) at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:170) at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:74) at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:88) at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:74) at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318) at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:107) at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318) at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:241) at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:155) at com.xpn.xwiki.XWiki.parseTemplate(XWiki.java:1415) at com.xpn.xwiki.web.Utils.parseTemplate(Utils.java:108) at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:199) at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431) at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236) at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196) at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414) at javax.servlet.http.HttpServlet.service(HttpServlet.java:690) at javax.servlet.http.HttpServlet.service(HttpServlet.java:803) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188) at com.xpn.xwiki.web.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:117) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:215) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:174) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:151) at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:874) at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665) at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528) at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81) at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:689) at java.lang.Thread.run(Thread.java:619)

At this time I only can watch the calendar if I switch to an empty month/year with the URL parameter month= or year=
I can fix the problem for some time if I klick "Save & View" at the Objects Editor.