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.