I added a WFS-NG data source pointing to an ArcGIS WFS service, dutifully added a few feature types, and started making WFS requests, such as:
However, the POST request (I set prefer POST over GET since ArcGIS objects to the use of plus sign in lieu of %20 when URL-encoding spaces) that GeoServer sent lacked the BBOX/PropertyName element.
Filling in the property name results in a successful response from ArcGIS:
After instrumenting GeoServer 2.14, it turned out that the BaseFeatureKvpRequestReader class handleBBOX method does not set propertyName with the default geometry of the feature type; consequently, when the POST request is built, the PropertyName element is left blank, as in the BBOXImpl class the e1 expression is an empty string.
I replicated the issue on the current master branch as well.