Comment 3 for bug 194496

This appears to be very similar to Bug #185589 (at least the title and initial portions of the report).

For the record, I have experienced this same issue using Xubuntu hardy. Selecting Applications > System > Users and Groups, and then Selecting "Unlock" produces the "cannot unlock user&groups: "could not authenticate. An unexpected error has occurred"" message.

$ dpkg -l gnome-system-tools | cat
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Installed/Config-f/Unpacked/Failed-cfg/Half-inst/t-aWait/T-pend
|/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err: uppercase=bad)
||/ Name Version Description
+++-==========================================-=============================-================================================
ii gnome-system-tools 2.21.92-0ubuntu1 Cross-platform configuration utilities for GNOME

$ dpkg -l policykit | cat
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Installed/Config-f/Unpacked/Failed-cfg/Half-inst/t-aWait/T-pend
|/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err: uppercase=bad)
||/ Name Version Description
+++-==========================================-=============================-=============================================================
ii policykit 0.7-2ubuntu4 framework for managing administrative policies and privileges