This indicates an attack attempt against an Arbitrary File Upload vulnerability in 3CX VoIP Phone System Manager.
The vulnerability is caused by an error when the vulnerable software handles a malicious HTTP request with a crafted URI. It allows a remote attacker to upload an arbitrary file via sending a crafted URI to a vulnerable application.
3CX Phone System v10
3CX Phone System v11
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.