This indicates an attack attempt to exploit a Denial of Service vulnerability in Apache Struts.
The vulnerability is due to insufficient validation of user supplied inputs when handling crafted URL. A remote attacker may be able to exploit this to perform a Denial of Service attack on a targeted system, via a crafted request.
Apache Software Foundation Struts 2 2.3.20 through 184.108.40.206
Apache Software Foundation Struts 2 2.5
Denial of Service: Remote attackers can crash vulnerable systems.
Refer to the vendor's advisory for updates: