Hi Greg,
Thanks for reporting this.
The problem seems to be that the Mapbox access token that Cesium uses is now invalid -- we'll look into getting a new one. The token is not intended for extended use, and it's possible that the quota for that token has been exceeded. When using the `MapboxImageryProvider` you can supply your own
token from Mapbox. Using your own token will look like this:
// Mapbox tile provider
var mapbox = new Cesium.MapboxImageryProvider({
mapId: 'mapbox.streets',
accessToken: 'thisIsMyAccessToken'
});
Erik