Shared calendars keep disappearing from our Exchange account
This is due to incorrect permissions set when setting up sharing in Exchange
Here is a post on Microsoft forums:
Essentially, if a user you enter into BusyCal's Sharing tab shows "No Access" or keeps dropping after a little while, this is an indication that the user has not given you read or write access to their calendar. There are various levels of permissions that can be configured when setting up sharing. Ensure that you have at least read permission to the user's calendar events / tasks.
Their proposed solution:
If you go into Active Directory Users and Computers, view the properties on one of the problematic accounts, click on the Security tab, click the Advanced button, the "Allow inheritable permissions..." checkbox needs to be checked. In our case, it was not checked, so we need to check it. After doing that, we then needed to remove and add back the full access delegate permissions using the Exchange Management Console.
Another user posted: We just found the resolution for this one as well. Although the mailbox had the proper credentials, apparently the calendar didn't. Once we added that admin account to the calendar, the error went away and the calendar entries are posting as they should.
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