This indicates an attack attempt to exploit a Cross-Site Scripting Vulnerability in NginxProxyManager.
This vulnerability is due to improper input validation for Proxy Host domain names. A remote, authenticated attacker could exploit this vulnerability by sending a crafted request to the target server. Successfully exploiting this vulnerability could result in arbitrary code execution in the context of the victim's browser.
NginxProxyManager prior to 2.9.17
System Compromise: Remote attackers can execute arbitrary script code within the context of the target user's browser.
Apply the most recent upgrade or patch from the vendor.