When combined with pam_ldap, to login to a centralised server, the GUI components of policykit fail, this means that the UI cannot shutdown, cannot mount disks, install packages, etc (Xubuntu 14.04).
I can provide any further info, but I don't know what you require to research this issue :).
pkexec from a terminal works fine.
This has been tracked down to being an issue with libpam-ldap
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