Stepdl, Sogo connector only does CardDAV needed for remote address books. Remote calendars (CalDAV) are supported by Lightning out of the box.
Girkers, no further idea, sorry, as I use egroupware instead of zarafa. But if I browse to the CalDAV address manually I don't get a single ics file but a page linking to multiple ics files, one for each appointment.
Do you have set up the correct address or protocol?