Comment 21 for bug 715707

Hello from 2015! Since this bug is still open I thought it might help if I post how I resolved the error on my Ubuntu 12.04 LTS headless server:

apt-get install python-gobject policykit-1

Edit XML configuration file /etc/dbus-1/system.d/com.ubuntu.ComputerJanitor.conf
and add following line as a child of the <policy context="default"> element: <allow send_destination="com.ubuntu.ComputerJanitor"/>

It should look like this
<!DOCTYPE busconfig PUBLIC
 "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
<busconfig>
        <policy user="root">
                <allow own="com.ubuntu.ComputerJanitor"/>
        </policy>

        <policy context="default">
                <allow send_interface="com.ubuntu.ComputerJanitor"/>
                <allow receive_interface="com.ubuntu.ComputerJanitor"
                       receive_sender="com.ubuntu.ComputerJanitor"/>
                <allow send_destination="com.ubuntu.ComputerJanitor"/>
        </policy>

</busconfig>