The "Unlock" button has no effect in the "Time and Date Settings" dialog

I am trying to change the timezone via UI.

In order to do that I run time-admin from a command line as a regular user. (It is also possible to open the same dialog by right-clicking on the Clock component on the panel, choosing the Properties menu item and then clicking on the Time and Date Settings... button.)

The following message appears in the console output:

Gtk-Message: 15:58:40.008: GtkDialog mapped without a transient parent. This is discouraged.

and the dialog window opens:

When I hover over the Unlock button, the tooltip appears: "Dialog is locked. Click to make changes".

If I click on the Unlock button then nothing happens on the screen, but an additional line appears in the console output:

** (time-admin:8856): WARNING **: 16:10:03.520: Error acquiring permission: Failed to acquire permission for action-id org.freedesktop.systemtoolsbackends.set

What does this message indicate and how to resolve this problem?

There is an exactly same question at Launchpad and a similar question at Ask Ubuntu, but both of them are unanswered.

The strange thing is that when I tried to create a new user and logged in to the system as that new user, the "Authenticate" window poped up for that user once I clicked the Unlock button.

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