Event ID 4 - Microsoft Exchange Control Panel couldn't proxy user

SCENARIO:

Error event from my Application log on a CAS server

From Googling - I have no exact match - either Event ID ref or Googling text from the Event description

Event ID 4  = Kerberos?

The events happen in isolation - I have two examples a week apart but that's out of my entire Event log, so I'm not feeling like it's red alert

No reports of a problem from users

Does any have any input as to what this Error relates to, or how I can troubleshoot this further?

Event ID 4

Message            : Current user: 'domain\example user'

                     Request for URL 'https://fqdn/ecp/default.aspx?exsvurl=1&mkt=en-GB'

                     failed with the following error:

                     Microsoft.Exchange.Management.ControlPanel.ProxyCantFindCasServerException: Mi

                     crosoft Exchange Control Panel couldn't proxy user ''X' on behalf

                     of 'Y'' from site 'London' to site 'Manchester'.

                     Services considered:

                    https://fqdn/ecp

                         Windows Authentication is disabled. Use 'Set-EcpVirtualDirectory -WindowsAuthentication:$true' to enable it.                         

Source             : MSExchange Control Panel