Note: They key to the bug is that when you create a SECOND admin user, Openfire adds a field that lists the authorized admins into the database including their JID (USERNAME@SERVERNAME).
When the Server Name is changed, however, Openfire doesn't update that field to the NEW Server Name, and so nobody can login.
If you never create a second admin user, Open Fire never adds that line to the database, and you'll never experience the issue even if you change the Server Name.
If you create a second admin user, but you never change the Server Name, you'll also never experience the issue.