We are no longer using WUnderground with OpenSprinkler. The current state does not allow much room for development/testing and is very inconsistent. We have decided to focus our efforts on OpenWeatherMaps.
The app is no longer exposing the key field as of 1.8.3. The weather service is also now ignoring the weather key. If you wish to go back to the prior state of things you could run your own local weather service based on a prior commit or possibly explore the solution written by @Peter. Details are here: https://opensprinkler.com/forums/topic/cutting-the-weather-underground-cord-homebrew-solution/