We're having the same problem, also 6.6.2. Setting the "correct" key isn't the awnser in our case.
To make it more fun, the Radius accounting to our Windows NPS/Radius server is working perfectly, even though we get these messages. We don't have any other Radius connections to/from the MSM controller.
When rebooting the messages dissapear for a few minutes, but sadly return if they never have left.
We're lacking the know-how to find what (or who) is causing these messages, looking at one of the many logs doesn't reveal us much.