This indicates an attack attempt to exploit Denial of Service vulnerability in Apache HTTP Server.
The vulnerability is due an error when affected application handles a crafted HTTP request. A remote attacker may be able to exploit this to cause a denial of service condition on the affected server.
Apache Software Foundation HTTP Server 2.4.1-2.4.9
Denial of Service: Remote attackers can crash vulnerable systems or services
Apply the most recent upgrade or patch from the vendor.