The Quartz Job configuration is (allegedly) invalid

From the logfile:

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2009-09-09 21:07:39,705 main INFO [atlassian.jira.scheduler.JiraSchedulerLauncher] Starting the JIRA Scheduler....
2009-09-09 21:07:39,720 main ERROR [com.atlassian.scheduler.OfBizJobStore] The Quartz Job class 'com.pyxis.greenhopper.jira.customfields.GreenHopperRankingJob' cannot be loaded
2009-09-09 21:07:39,720 main ERROR [com.atlassian.scheduler.OfBizJobStore] The Quartz Job configuration for 'GreenHopperIndexes' is invalid! This can be caused by Plugin JARS no longer being available on the class path.
2009-09-09 21:07:39,720 main ERROR [com.atlassian.scheduler.OfBizJobStore] A NoOp Job class has been returned and hence 'GreenHopperIndexes' will not do anything until fixed.
2009-09-09 21:07:39,741 main INFO [atlassian.jira.scheduler.JiraSchedulerLauncher] JIRA Scheduler started.