You only have to backup the system state with ntbackup. The message will disappear and remind you after 90 days.
If you are using Veritas Backup Exec to backup your server and you are backing up the "System State" you can ignore the message according to Veritas. See “Veritas Support Document ID: 282890”.
This event is logged when a partition is not backed up during the backup latency interval. Only one event error message is logged each day for each partition that a domain controller hosts. See ME914034
for additional information about this event.
Information about this event can be found in the following articles: "Active Directory Replication Tools and Settings", "How the Active Directory Replication Model Works", "How Active Directory Application Mode Works", "Introduction to Administering Active Directory Backup and Restore", and "How the Data Store Works".