It is almost fixed !
I did the following :
I registered a caldav account on my usual server. the calendar was not seen. I set it as default, I imported the ics file from this calendar (I downloaded it manually), then I saw my events in the calendar - expected behavior - but now, all new events created on N9 are synced on the caldav server.
Unfortunately, events created by other devices and stored on the caldav server are not seen by N9...