In the URL of the loading Story Map builder page, there are two '/' on the right of the organization URL section. [E.g.: <a href="https://ess.maps.arcgis.com//apps/MapSeries/index.html" target="_blank">https://ess.maps.arcgis.com//apps/MapSeries/index.html</a>?..]Remove one of the slashes. Once the '/' is removed, the builder launches.