This indicates an attack attempt against a Cross-Site Scripting vulnerability in Nextcloud Server.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. A remote attacker may be able to exploit this to execute arbitrary script code within the context of the application.
Nextcloud Server prior to 20.0.6
System Compromise: Remote attackers can execute arbitrary script code in the context of the affected site.
Apply the latest update from the vendor.