Dear Maintainer,

   * What led up to the situation?

Open gnome-control-center.

   * What exactly did you do (or not do) that was effective (or
     ineffective)?

Click the icon of a panel that requires privileges, e.g. Date & Time or
Users. Most of the panel is grayed out, which is expected.

Click on the Unlock button.

   * What was the outcome of this action?

Nothing happens but the following line is printed on std{out|err}:

   (gnome-control-center:4316): Gtk-WARNING **: Error acquiring permission: Failed to acquire permission for action-id org.gnome.controlcenter.datetime.configure

   * What outcome did you expect instead?

Being asked for my credentials and able to use the panel after entering
them.

Dear Maintainer,

I'm seeing the same problem here with printer or user settings.
This appears in journalctl

nov. 28 09:38:15 imac gnome-session[2584]: AuthenticationDialog<.performAuthentication@resource:///org/gnome/shell/ui/components/polkitAgent.js:224
nov. 28 09:38:15 imac gnome-session[2584]: wrapper@resource:///org/gnome/gjs/modules/lang.js:178

and this appears when launching gnome-control-center, going to user settings, click unlock,
then close gnome-control-center window:

(gnome-control-center:6383): GLib-GIO-CRITICAL **: g_permission_acquire_finish: assertion 'G_IS_PERMISSION (permission)' failed
Erreur de segmentation

Regards,
Jérémy

Same with a fresh debian 9.6:

Clicking 'Unlock' does nothing, console shows:

(gnome-control-center:11030): Gtk-WARNING **: Error acquiring
permission: Failed to acquire permission for action-id
org.gnome.controlcenter.datetime.configure

ii  policykit-1		0.105-18+deb9u1	i386
ii policykit-1-gnome	0.105-6		i386

Let us know if you need more info.