getWeatherFeature("yesterday") fails

I’ve recently started getting these errors twice daily, every time my application checks if sprinklers need to run:

getWeatherFeature(‘yesterday’): [response:[error:[description:a requested feature is not valid due to exceeding rate plan, type:invalidfeature], features:[:], termsofService:, version:0.1]]

Should I assume ST cannot guarantee their plan can cover the current load, and revert to a different mechanism - or to my own wunderground account?