It seems there's a problem with a specific dropdown-menu in Lightning, that appears when a new Event is created, and privacy settings are being chosen in the event-properties window, by clicking the arrow next to the Privacy icon.

A dropdown-menu opens up, and lists three options:

Public Event
Show Time and Date Only //imho this option should be named 'confidential'
Private Event.

We configured the SOGo Server in the following way:

Public Event means:
Show all Information to all authenticated Users

Confidential Event means:
Hide all Information about this Event from all authenticated Users.

Private Event means:
Show Time and Date Only to all authenticated Users.

The problem is, that calling the option in the dropdown-menu
'Show Time and Date Only' instead of 'Confidential' already implies
a specific privacy policy, although other settings could have been configured.