The cron job "Activity processor (Hourly)" has started failing for no apparent reason with the message "Failure, Error message: Process Activities failed" in the job log, although the message coming from cron is "Failed Processing: . Reason: DB Error: syntax error". Neither message is much help.
We are running CiviCRM 4.5.8 under WordPress 4.2.2.
Has anyone experienced this? Where do I start to find and fix the problem?