That is probably how the calendar is marked for sharing on the caldav server. Just how did you "import" it? Generally you connect to the caldav server with a URL link and it serves the calendar data. But people do find other ways to do it.

Thank you for your response. The problem is that it worked before we closed the 5000 port on the router because of a cyber attack. I reimported the calender data from an ics file but this did not change anything. The calender accounts of my colleagues which are on the same server show messages with text.