[otrs] Ticket reminders on closed tickets

Hi, I have an issue here with tickets that are in a closed state, but a reminder is being sent via mail to the agent. All tickets have had a reminder set and were (and are) in closed status. The tickets do not appear in any queue or in the locked tickets. When I click on the link in the reminder I can see that the ticket is closed successful and in normal state. If I now lock it again and close it, it will send another reminder in a couple of days. Anyone any idea? Am I forgetting to run a cron job or something? Regards, Arnold Ligtvoet