This indicates an attack attempt to exploit an Insecure Deserialization in Spring Framework.
The vulnerability is due to insufficient validation of user supplied inputs. Successful exploitation could result in remote code execution.
Spring Framework 5.3.0 to 5.3.17
5.2.0 to 5.2.19
Older, unsupported versions are also affected
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.