Someone may be trying to update your DNS zone using the RFC 2136 Dynamic Update protocol, but they may not have permission to do so.
Windows 2000 machines have a habit of sending dynamic update requests to DNS servers without being specifically configured to do so. If the update requests are coming from a Windows 2000 machine, see https://support.microsoft.com/en-us/help/246804/how-to-enable-or-disable-dns-updates-in-windows-2000-and-in-windows-se for information about how to turn them off.
If you get this log message when trying to perform a DNS update with a program such as nsupdate, please check your named configuration.