This indicates an attack attempt to exploit a Remote Code Execution vulnerability in Plex Media Server.
The vulnerability is due to an error when the vulnerable software handles a maliciously crafted HTTP request. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application.
Plex Media Server version 1.19.2 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Applied latest upgrade or patch from the vendor.