This indicates an attack attempt to exploit a Denial of Service Vulnerability in NTPsec ntpd.
A remote user can exploit this vulnerability by sending a crafted packet with a valid authentication key to the target server. Successful exploitation could lead to the application crash and denial of service condition.
NTPsec ntpd prior to 1.1.3
Denial of Service: Remote attackers can crash vulnerable systems.
Refer to the vendor's advisory for updates: