This indicates an attack attempt to exploit a Insecure Deserialization vulnerability in Microsoft SharePoint.
A remote attacker may be able to exploit this vulnerability to execute arbitrary code within the context of the application, via a crafted request containing a marshalled serialized object to the vulnerable server.
Microsoft SharePoint Enterprise Server 2016
Microsoft SharePoint Server 2019
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.