CalDAV - Calendar - Wrong Appointments

Hi There

We use the Nextcloud in combination with CALDAV and Outlook 2016. Every person has a individual permission to access the calendar inside the nextcloud (share function but without writing permission).

Our problem is, that if person 1 places an appointment in his calendar, person 2, whos not releated to that appointment in anyway, gets the appointment listet in his calendar.

Any ideas why that happens ?

Greetings Tiaca