When you say 'log in' you are trying to connect to it through the GUI? It doesn't support that. You can access it via the CLI and there is a rich thread on the commands at http://docs.splunk.com/Documentation/Splunk/latest/Admin/CLIadmincommands (thanks Drainy 😉
Was the admin's default password of "changeme" ever changed? If you can not figure out the password, then
Now you have the default credentials (admin/changeme) back.
However, if you want to achieve similar results to clean all, then: