While I am not totally sure of this behavior, it would seem to me that if the user went on vacation after the task was in their list, and the task was subsequently acted on by another user, then clicking on the email link after the task has moved on would produce this message. Given that the email was generated when the task was valid it has no way of knowing that the task has been completed.
Also, did the user set their Out Of Office in DocuWare? If they do set their Out of Office, tasks will not be assigned to them for the duration.
Also, are you using substution lists?
Senior Director Support Americas