Hi Bruce,
notification_user_map is the table name, and sent_date is the column name. Since the sweep proc only processes notifications that have not already been sent (ie , sent_date is null), you can stop it from processing the ones that are stuck in the queue but setting sent_date to any valid date (I am not sure of the syntax for that in Postgres, as I work mostly in Oracle and times/dates is one area where they differ).
This will only fix the problem if you are right that you have old messages stuck in the queue. If you do this, then create a new notification and get the same error, then something else is wrong.
