com.zimbra.common.service.ServiceException: invalid request: missing required element: link
ExceptionId:sync-mbox-xxxx@xxxx.xxx:1390810286849:d0bb182b8379337f
Code:service.INVALID_REQUEST
at com.zimbra.common.service.ServiceException.INVALID _REQUEST(ServiceException.java:260)
at com.zimbra.common.soap.Element.checkNull(Element.j ava:306)
at com.zimbra.common.soap.Element.getElement(Element. java:199)
at com.zimbra.cs.mailbox.PushChanges.sendRequest(Push Changes.java:576)
at com.zimbra.cs.mailbox.PushChanges.pushRequest(Push Changes.java:549)
at com.zimbra.cs.mailbox.PushChanges.syncFolder(PushC hanges.java:728)
at com.zimbra.cs.mailbox.PushChanges.sync(PushChanges .java:246)
at com.zimbra.cs.mailbox.PushChanges.sync(PushChanges .java:195)
at com.zimbra.cs.mailbox.MailboxSync.sync(MailboxSync .java:193)
at com.zimbra.cs.mailbox.ZcsMailbox.sync(ZcsMailbox.j ava:124)
at com.zimbra.cs.mailbox.ZcsMailbox.syncOnTimer(ZcsMa ilbox.java:105)
at com.zimbra.cs.mailbox.SyncMailbox$1.run(SyncMailbo x.java:279)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)

What is the cause of this error?

Thanks