A few days ago, I happened to run into this issue myself. The root cause is, when installing Ranger, in the "External URL" property, the administrator entered "http://hostname.example.com:6080/", instead of the expected "http://hostname.example.com:6080" (WITHOUT the trailing slash character). Even though the Ranger installation would go through, Ranger's Usersync would log errors in /var/log/ranger/usersync/usersync.log due to this extraneous character. Also, any attempt to enable any of Ranger's plugins would fail, with the error message "Ambari admin username and password are blank", because Ranger is indeed missing many users, including the important one amb_ranger_admin. To fix this, just edit this property and remove any character after port 6080, and everything will start working.
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