FullAccess fails with the error: The specified folder could not be found in the store.

I have been banging my head against the Exchange 2007 brick wall for the last month over the error: "The specified folder could not be found in the store."

Sumatra's conference room analysis tool's Exchange Web Service calendar folder "FindItem" request failed for about 5% of the rooms at one client. Other clients do not have this problem! The service account had FullAccess to all rooms. All conference rooms were on the same Exchange mailbox server, in the same OU, configured to autoaccept. Some had delegates, some did not.

The client could use the service account credentials to access the calendars via OWA. Was it a corrupted meeting? We changed FindItem's interval. No luck. Was EWS timing out over a large mailbox? Increased the HTTP timeout. No luck. Our FindItem requet uses the DistinguishedFolderID. We called GetFolder to find the FolderID. It failed on the inbox with the message "The specified object was not found in the store", and for the calendar folder with the message "The specified folder could not be found in the store".

Ahha! The permissions were not inherited. We added "InheritanceType: All" and it worked. Here is the syntax:

Get-Mailbox -filter {isResource -eq $True} -Resultsize unlimited
Add-MailboxPermission -User: xxxx -AccessRights: FullAccess
-InheritanceType: All