My guess is that the error is coming from the service (not the proxy), so might not be an issue with how you configured the proxy.You can get this error back from the actual service (not the proxy), if for example you click where that sample service does not have data coverage, like in the ocean or outside the US. http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/Network/USA/NAServer only covers USA...What does the detailed error say?