Comment 4 for bug 983879

I'm having the same (or a similar) issue.
I even did a fresh install of Xubuntu (for remotely related reasons), with no change in behaviour.
Here is what I observed:

The taskbar menu will show all normal menu entries (lock screen, standby, hibernate, restart, shutdown, logout).
Clicking hibernate will report failure due to "not authorized". The exact message is: "Herunterfahren fehlgeschlagen", "Kann Sitzung nicht in Ruhemodus versetzen", "not authorized". (Note that the first two messages are translated and have proper capitalization, but "not authorized" does not have either. Looks like a message from somewhere deep down that's simply passed through - trouble is that the dialog doesn't tell me where that message came from, so I can't find the true cause.)

xfce4-session-logout --hibernate will show exactly the same message dialog.

sudo xfce4-session-logout --hibernate will give "Fehler beim Abmelden" "Keine Antwort von der Sitzungsverwaltung erhalten" "The name org.xfce.SessionManager was not provided by any .service files". (Rough translations: "Error logging out" "No response received from the session manager".)

If this is really a general problem, Bug #992880 is a duplicate.