Comment 3 for bug 220697

I've tried with another user belonging to the admin group, with the same result. This machine was upgraded from Gutsy.

FYI, this is the contents of /usr/share/PolicyKit/policy/system-tools-backends.policy if it's of any use:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE policyconfig PUBLIC
 "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/PolicyKit/1.0/policyconfig.dtd">

<policyconfig>

  <action id="org.freedesktop.systemtoolsbackends.set">
    <description>Manage system configuration</description>
    <message>System policy prevents modifying the configuration</message>
    <defaults>
      <allow_inactive>no</allow_inactive>
      <allow_active>auth_admin</allow_active>
    </defaults>
  </action>

  <action id="org.freedesktop.systemtoolsbackends.self.set">
    <description>Change user configuration</description>
    <message>System policy prevents modifying the user configuration</message>
    <defaults>
      <allow_inactive>no</allow_inactive>
      <allow_active>auth_self</allow_active>
    </defaults>
  </action>
</policyconfig>

I'm sure my Policykit configuration is correct.