Looks like this request is triggered by the activity map feature in AEM and is expected when the editor loads, 404 occurs because the config. path doesn’t exist unless activity map is fully set up. if you don’t plan to use activity map, you can disable the cq.activitymap client library to stop these calls. if you do plan to use it, configure the adobe analytics integration with activity map. since this behavior persists in AEM 6.5 it appears to be a known issue.
if disabling or configuring doesn’t resolve it, pls raise an adobe support ticket.