After a mailbox is moved from Microsoft Exchange Server 2007 or Exchange Server 2010 to Exchange Server 2013 or Exchange Server 2016, users who have Author permissions for a shared calendar of the mailbox cannot edit existing items that they created. Additionally, they receive the following error message:
Your changes cannot be saved because you don't have permission to modify some or all of the items in this folder. Do you want to save a copy of this item in the default folder for the item?
Note Users can edit items that were created after the mailbox was moved.
When a user who has Author permissions tries to open a calendar item, Exchange Server determines whether the user is the author of the item. Exchange Server does this by comparing the SID that is saved in PR_CREATOR_SID of the item with the SID of the user who is trying to open the item. When a mailbox is moved from Exchange Server 2007 or Exchange Server 2010 to Exchange Server 2013 or Exchange Server 2016, PR_CREATOR_SID values for items are not migrated, and the values are set to NULL.
This is a known issue in Exchange Server. To work around this issue, change the permissions of users for the shared calendar from Author to Editor. Then, those users will be able to edit calendar items.