Thanks Stan for your feedback.
Actually it seems I've narrowed the origin of the problem.
I have been using Mate as DE for many months without any problem.
Only recently (when still in F27 and some days before the start of the problems described in this thread) I switched from gdm to lghtdm because I had some lagging events and I tried to eliminate at all gnome/gdm/gnome-shell dependency

Now (I'm in F28) it is simply sufficient to re-enable gdm and connectusing Gnome as DE to see that all is ok:
- logout/shutdown buttons without problems (previously a logout originated an automatic relogin...)
- switch wifi on/off and other components (such as external usb disks) without getting authorization errors
- audio adapter is seen ok with its analog and hdmi settings (and it works too... ;-)

So it seems that the problem doesn't show with default Fedora environment but only with the combination I outlined above.
I also tried Cinnamon (with lightdm) and same problems.

Possibly gdm in its start enables some permission forĀ  normal users that lightdm (at least in latest updates) doesn't provide?
I'm going also to test other combinations, to narrow down better (eg lightdm with gnome DE session, ecc...)

Gianluca