I'm built an UWP Application to generate Routes.
In Windows Store (Tablets or Desktops) the LocalRouteTask awalys work as well, but in Windows Phone 8.1/10 i got an intermittent issue. Sometimes when my application (Windows Phone 8.1/10) will instance the LocalRouteTask i got the follow Exception: "JSON-content is empty.:". But how i said, that's occurs at intermittent times.
I attached a print and bellow a link to download a sample application code, when you can got a issue.
PS: I had test on Lumia 520, Lumia 930 and All emulators.