Not at all sure but here are a few suggestions
In /etc/hosts change the first lines to
127.0.0.1 localhost <machinename>
127.0.1.1 <machinename>
The second line should already contain <machinename> so you just add it to the first line
Note hosts is a very important file and you should not play around with it
Run groups in a terminal - this shows what groups you are a member of. If you are the first user you should be a member of admin
Also check /etc/PolicyKit/PolicyKit.conf there should be a line
<define_admin_auth group="admin"/>
in there between
</match>
and
</config>