This indicates an attack attempt to exploit a Remote Code Execution vulnerability in DotNetNuke.
The vulnerability is due to a deserialization error when the vulnerable software handles a maliciously crafted HTTP request. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted HTTP request.
DotNetNuke before 9.1.1
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.