A free shared mongodb cluster has too many connections opened which prevents new connections from working. We are looking into which user(s) are opening too many connections and we will start a new cluster to alleviate the issue. We have no immediate solution, sorry for the inconvenience.
08:40: The problem has been alleviated by allowing more connections. It will slow down the service but you can at least connect to your databases and migrate to paid add-ons if you were using this service for production. We will start a new cluster very soon to improve performance.