The VOEvent messages produced by the Alerter have a bug in the XML. In <ObservationLocation><AstroCoordSystem> the attribute is idref and should be id. Strictly speaking it should be of the form:
<ObservationLocation>
<AstroCoordSystem id="UTC-FK5-TOPO" xlink:type="simple" xlink:href="ivo://STClib/CoordSys#UTC-FK5-TOPO/" />
<AstroCoords coord_system_id="UTC-FK5-TOPO">