Error when trying to send appointment using Icalendar

I receive the following ical error when trying to send appointment request Error in execution of monitored action 'iCalendarModule.SE_Appointment_ProcessQueue' (execution id: 5e3ff959-1142-4947-be65-bc4f948a913e, execution type: SCHEDULED_EVENT).  NoClassDefFoundError occurred for class 'Could not initialize class net.fortuna.ical4j.model.CalendarDateFormatFactory'. Please make sure the userlib folder of  your project includes the correct library which contains this class.           
1 answers

Hi Eben,

   This looks like a conflict in your java userlib directory.  When this happens, it usually means that there are multiple versions of the same jar file in the userlib directory, one of which has classes you need and one of which does not.  The Java class loader only loads up one version of the jar file and therefore the class appears to be missing when you try to execute your action.  These problems can be a little tricky to debug because the content of your userlib folder varies depending on what app store modules you have downloaded.

  The first thing I would try is to take a look at your userlib folder and look for duplicate jars (jars that have the same name but different version numbers) and delete the lowered-number version.  Then redeploy and see if your action can run.