Mail Threading not working on j.a.com

These are the exceptions in the logs:

2004-02-21 23:29:30,229 WARN [atlassian.jira.mail.MailThreadManagerImpl] Error retrieving mail thread: Passed List had more than one value.
java.lang.IllegalArgumentException: Passed List had more than one value.
at org.ofbiz.core.entity.EntityUtil.getOnly(EntityUtil.java:58)
at com.atlassian.jira.mail.MailThreadManagerImpl.threadNotificationEmail(MailThreadManagerImpl.java:74)
at com.atlassian.jira.mail.JiraMailThreader.threadEmail(JiraMailThreader.java:42)
at com.atlassian.mail.queue.SingleMailQueueItem.send(SingleMailQueueItem.java:37)
at com.atlassian.mail.queue.MailQueueImpl.sendBuffer(MailQueueImpl.java:55)
at com.atlassian.jira.service.services.mail.MailQueueService.run(MailQueueService.java:23)
at com.atlassian.jira.service.JiraServiceContainer.run(JiraServiceContainer.java:51)
at com.atlassian.jira.service.ServiceRunner.execute(ServiceRunner.java:50)
at org.quartz.core.JobRunShell.run(JobRunShell.java:147)
at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:387)