iCloud Sync with Zoho Calendar (Invitations)

iCloud Sync with Zoho Calendar (Invitations)

Hello, everybody,

today we set up Zoho Calendar for our team and imported the account via calDAV at Apple (MacBook).

The appointments are displayed, but I can't confirm appointment invitations via Apple Calendar. When I click on "Accept", the appointment is displayed briefly as "Confirmed", then within a second it is displayed again as "Not confirmed".

Is the problem known? What do I do here?