OpenLayers 3.5 WFS-T update to GeoServer 2.7 raises StringIndexOutOfBoundsException

This is because boundedBy / bbox is a property that OpenLayers creates when parsing the GML, it is not a real feature property. So when you update, you should clone the feature and get rid of it. Some example code:

// do a WFS transaction to update the geometry
var properties = feature.getProperties();
// get rid of bbox which is not a real property
delete properties.bbox;
var clone = new ol.Feature(properties);
clone.setId(fid);

If you need more example code, here is the code we use in the OpenGeo Suite SDK for WFS-T: https://github.com/boundlessgeo/suite/blob/master/sdk/src/main/resources/client/ol3-common/app/TransactionHandler.js#L105:L110