If you are getting this it means you must be using the Map Quest Open Street Map tile server. That server was shutdown on 7/11/2016.
Maybe you are using the OpenStreetMapQuestSource class provided by Telerik?
You will need to switch map providers. You might try MapBox, you will need to have an account and you may need to create a custom map provider and source.
We created a TitleProovider that allows us to try multiple services by swapping out the TiledMapSource, so when MapQuest shutdown their service, we moved to Mapbox which we had already implemented.
Here is the bit of our MapBoxSimpleItsSource class that does the work:
zoomLevel = ConvertTileToZoomLevel(tileLevel);