Robert,
please refer the URL's for both success and failure cases.
Failed Coordinates(Not getting address):
https://<My 10.5.1 Server>/arcgis/rest/services/AddressLocator/GeocodeServer/reverseGeocode?outSR=%7B%22wkid%22%3A4326%7D&location=%7B%22x%22%3A77.36622486425071%2C%22y%22%3A28.70099093986658%2C%22spatialReference%22%3A%7B%22wkid%22%3A4326%7D%7D&distance=100&f=json
https://<My 10.5.1 Server>/arcgis/rest/services/AddressLocator/GeocodeServer/reverseGeocode?outSR=%7B%22wkid%22%3A4326%7D&location=%7B%22x%22%3A77.36194183444022%2C%22y%22%3A28.775230123248484%2C%22spatialReference%22%3A%7B%22wkid%22%3A4326%7D%7D&distance=100&f=json
Passed Coordinates(getting the adresses):
https://<My 10.5.1 Server>/arcgis/rest/services/AddressLocator/GeocodeServer/reverseGeocode?outSR=%7B%22wkid%22%3A4326%7D&location=%7B%22x%22%3A77.14707650561374%2C%22y%22%3A28.76309487211875%2C%22spatialReference%22%3A%7B%22wkid%22%3A4326%7D%7D&distance=100&f=json
Thanks & Regards
Siva