This indicates an attack attempt against a Authentication Bypass vulnerability in DotNetNuke.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application allowing an attacker to escalate their privileges to the security context of the user they are targeting. As a result, an attacker may be able to execute arbitrary code within the security context of the targeted user
Above DotNetNuke 6.0.0
Security Bypass: Remote attackers can bypass security checking of vulnerable systems.
Refer to the vendor's website for suggested workaround.