AifMessageInspector error on AOS server

I've seen this error in the windows event log now in several AX 2012R2 environments and would like to know what it means and how to troubleshoot it further. Any help appreciated:

Object Server 01:  An error has occurred in the services framework.  Method: AifMessageInspector::AfterReceiveRequest.  Error: System.ServiceModel.FaultException: Failed to logon to Microsoft Dynamics AX.

  at Microsoft.Dynamics.Ax.Services.AxServiceOperationContext.InitializeSession()

  at Microsoft.Dynamics.Ax.Services.AxServiceOperationContext.InitializeContext()

  at Microsoft.Dynamics.Ax.Services.AxServiceOperationContext.Attach(OperationContext owner)

  at System.ServiceModel.ExtensionCollection`1.InsertItem(Int32 index, IExtension`1 item)

  at System.Collections.Generic.SynchronizedCollection`1.Add(T item)

  at Microsoft.Dynamics.Ax.Services.AifMessageInspector.AfterReceiveRequest(Message& request, IClientChannel channel, InstanceContext instanceContext)