Here is a list of things you can do to troubleshoot calendar permissions not syncing in Exchange:
- Check whether firewall/routers are blocking traffic from Exchange to other endpoints.
- Make sure the latest versions of Exchange and Outlook are both installed.
- Check the service account set up by your IT administrator.
- Reset a user's Outlook client.
- Check if user's mailbox is in the enabled state.
- Confirm that the EWS MAPI virtual directory has the correct 'authFlags' set.