I see this problem with system-config-users but I can also reproduce
it manually in the Python interpreter.  In s-c-users, select an
existing user and then click on the "Properties" button in the
toolbar.  Then change the username to something else.  Then hit "OK".
 The following message gets printed out on the command line:

Traceback (most recent call last):
  File "./userProperties.py", line 512, in on_ok_button_clicked
    self.parent.ADMIN.modifyUser(self.userEnt)
RuntimeError: unknown error

If you exit s-c-users, you'll see that libuser actually did make the
correct changes, but the bindings are throwing a traceback for some
reason.