geoserver - Openlayers: error 405 (Method not allowed) while attempting to add a WFS-T layer

I need your help: basically I need to add, on a map with several layers already on, another layer WFS-T to allow adding and saving new features to a database. I checked on the internet and syntax looks OK, but all I get is an error 405 (method not allowed).

The server I'm using for the web app is IIS Express; for the layers I'm using GeoServer hosted on it's default server (not Apache) on port 8080. Of course I'm already using a proxy. Here is also the piece of my code about the WFS layer:

var database_poi = new OpenLayers.Layer.Vector("Points on Database", {
            protocol: new OpenLayers.Protocol.WFS({
                version: '1.1.0',

                url: 'http://localhost:8080/geoserver/wfs',
                featureType: 'database_poi',
                geometryName: 'ogr_geometry',
                srsName: 'EPSG:4326',
                featurePrefix: "Grottole"
            }),
            strategies: [new OpenLayers.Strategy.Fixed(), saveStrategy],
            projection: new OpenLayers.Projection('EPSG:4326'),
            eventListeners: {
                'featureselected': function (evt) {

                    var feature = evt.feature;
                    var popup = new OpenLayers.Popup.FramedCloud("popup",
                        OpenLayers.LonLat.fromString(feature.geometry.toShortString()),
                        null,
                        feature.attributes.ogr_geometry,
                        null,
                        true,
                        null
                    );
                    popup.autoSize = true;

                    popup.maxSize = new OpenLayers.Size(400, 800);
                    popup.fixedRelativePosition = true;
                    feature.popup = popup;
                    map.addPopup(popup);
                }
            }

        });

I really hope you can help me