Why is "Authentication is required to ...?"
If you can get a Live CD / Live USB for the version of Ubuntu you're running, boot to it and select the "Refresh system files" option. This may reset some other changes you have made to system files.
For me, the problem went away when I
do-release-upgraded that system from 15.10 to 16.04 -- probably because thepolkitinfrastructure changed and so did the configuration causing the issues.
Unfortunately, I don't know of another way to fix this without Live Media or waiting for a version change.