Plugins have received their custom subdomain for Storyblok so they no longer can access the app.storyblok.com context from within the field type. This was necessary to not allow field types to access localstorage and other app.storyblok.com information that are not intended to be shared with the plugins.

Please update your configurations to allow plugins.storyblok.com rather than app.storyblok.com since all API calls will be done by that domain starting with 30.04.2019.