Missing shared calendar from specific user

Hello Nextcloud Community,
i have a problem and am slowly despairing about it.
We have 4 users, one is sharing a calendar and three should be able to edit em.
The calendar ist shared but he does not display in any of the three accounts. I’ve already reshared the calendar with read/write permissions - nothing happend. (If i try to share the calendar in Outlook it displays the users-own calendar) The users a confiured through an LDAP-Server. It worked till a few months and than it stopped without a change. I’ve testes with other users if the calendar-share even works - it does. But this calendar is different. Do you have any ideas? And another question: Do i have as admin no permissions to edit or even check shared elements like calendar?
Thanks in advance.