Im my case, it was “uBlock Origin” Adblocker in Firefox that prevented (only) the “share icon” of currently unshared calendars to be shown. After whitelisting my NC server, not only the “already shared” calendars could be edited but “unshared” calendars also my be shared (icon is now visible).
Devs/Mods: Isn’t this “Adblocker” issue worth an FAQ entry?
Since this is browser independent, the "[Chrome] " tag of this thread should be removed.
Crosslink: Share a calendar or adressbook is not possible after upgrade