Check Diameter Message Validation in the Error Resolution appendix of the Policy and Charging Application User Guide to investigate and understand the circumstances where the error occurs.
Go to the Policy DRA SOAM GUI at Main Menu > Alarms & Events > View History and set the Display Filter by Events (in particular, Event 22700 - Protocol errors in Diameter Requests.
Use the Origin-Host value of the received Request found in Event 22700 - Protocol errors in Diameter Requests to understand from where the Request was sent.
Get the measurement report from Main Menu > Measurements > Report for, but not limited to, "Diameter Exception," "DSR Application Exception," and "Policy DRA Diameter Exception" Measurement Groups.