I have ArcGIS 10.1 installed with the web adapter for IIS. I've added a config file to the root of IIS7 to enable Cross Origin Resource Sharing (CORS) as per this page. I've pushed the cors enabled ArcGIS server into the list of default cors servers as per this page:
"Origin http://vmagsten is not allowed by Access-Control-Allow-Origin.".
Edit: And the feature layer fails to load. I have also tried loading a dynamic layer and this too fails with the same error _557 (see image)
Edit: I should not be seeing this error as the above steps should mean that the server does support cross origin. This statement by esri that this error can be ignored does not apply in this case since this server should support cors. It does mean that the first error to the ESRI server can be ignored.
Edit: here is a sample response from a query on this layer showing that the response has Access-Control-Allow-Origin