We have seen a couple of scenarios where the calendar permission does not work as expected.
Example: User A has Editor permission on User B’s calendar however he is not able edit the calendar items via OWA.
In most cases removing all the calendar permissions and re-adding it fixes the issue.
It becomes very difficult to note down all the permissions, remove it and add it manually.
We can use below script to remove and re-add the calendar permission for a given user.
This script would work for Exchange Server 2010, Exchange Server 2013 and Exchange Server 2016 but would not work for Exchange Server 2007 as cmdlets Add-MailboxFolderPermission and Remove-MailboxFolderPermission was not included in Exchange Server 2007.