I hope you already find your answer, but there is a way to set user credentials for the Ranger admin user interface without using the web interface.
In the install.properties file in the ranger-admin directory, add the following property
rangerAdmin_password=<your_custom_password>
Now run ./setup.sh in your ranger-admin directory. Then you can log in to Ranger Admin (i.e. http: // localhost: 6080 ) using
Username: admin
Password: your_user password
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