You are using Folder Permissions Manager to apply changes to delegates on a specific mailbox, but the changes fail to apply with this error message:
The logon account does not have the appropriate permissions to complete the operation on mailbox 'User Name'.
This is a known issue with Microsoft Exchange Web Services (EWS).
Folder Permissions Manager uses EWS to update delegates. EWS fails to apply changes when any existing delegates are orphaned (the associated Active Directory accounts no longer exist). In this case EWS returns (500) Internal Server Error.
To solve the problem, the workaround is to open the affected mailbox in Outlook and manually remove the delegates that are orphaned (the associated Active Directory accounts no longer exist). Delegates on the affected mailbox can now be managed using Folder Permissions Manager.
Folder Permissions Manager, version 8 and later.