Unfortunately there is a bug in the management portal that corrupts a NH authorization rules. It will be solved very soon. In the meantime, you can create a new NH from a Console App and Service Bus preview dll with the following code:
NamespaceManager mgr = NamespaceManager.CreateFromConnectionString("<connection string from your namespace>");
var hub = new NotificationHubDescription("myhub");
hub.GcmCredential = new GcmCredential("<your Google API Key>");
mgr.CreateNotificationHub(hub);
Make sure to use the connection string retrieved from the portal in the following way: 1) Click Service Bus on the left pane 2) On the right-pane, select your namespace, then click Connection Information 3) Copy the Connection String for RootAccess, as shown below:
Sorry for the inconvenience.