I am using ArcGIS Qt SDK 10.2.6. We made a GPK of Viewshed Analysis tool and using it in Qt Application. What we observed is that sometimes output of gpk is not coming in Qt Application wheras for same inputs Viewshed is working fine in ArcMap. We ensured service of gpk is running.
Can you try opening the service endpoint in a browser and see if you can execute the REST request manually from there?