regarding the loading icon. Currently you would have to change the logo in ui_app.jade that is used while loading. Though that looks like code duplication for me, so I will check if we can remove that second place in the future.
Regarding the login error: Could you provide any additional logs, that are logged at the Kibana server (on the console of the running Kibana process)? There should be a stacktrace there, that provided additional information.