Varnish exception while retrieving icons

I’m seeing a Varnish 503 error while retrieving icons listed in stations.kml.
E.g. for https://tfl.gov.uk/tfl-global/images/syndication/roundel-overground.png , I’m getting:

{
  "exceptionType" : "VarnishBackendSickException", 
  "httpStatus" : "Service Unavailable", 
  "httpStatusCode" : "503", 
  "message" : "This service is currently unavailable", 
  "relativeUri" : "/tfl-global/images/syndication/roundel-overground.png" 
}
				
Thanks,
Ricardo