Hello @Romuald_Pochet,
For information, we discussed calendar sharing a bit here [Caldav] Configuration multicomptes but it’s more for internal sharing.
Regarding your issue, since the CalDAV protocol is quite complex, not everything may be perfectly handled yet.
I will try to reproduce your case to see if a fix is possible. Is it a Google account sharing a calendar with another Google account? When you click on « Synchronize » on the CalDAV integration configuration page, do you get an error?