Per this service change notice by NOAA/NWS…
http://www.nws.noaa.gov/om/notification/scn16-16wng.htm
The change was supposed to be effective June 15, 2016, but actually went effective the week of August 22nd, 2016. The URLs for its data services have changed. The URL for Level 3 polling is hard-coded into the software, and in order to continue receiving data, you must upgrade to GRLevel3 2.50 or higher. An alternative would be to subscribe to AllisonHouse, but be warned not all features will continue working.
See this FAQ item for the latest GR versions, and how to obtain them.