Cannot delete items in Public Folder "Outlook Security Settings"

I am working with the Outlook Security Template which needs to be stored in the Top-level public folder called "Outlook Security Settings". There are now a few defunct documents in there that are no longer needed. When I try to delete them I get the error message:
"The item could not be deleted. It was either moved or already deleted, or access was denied"

When deleting the documents, I've made sure Outlook was not in cached mode.
The account I'm using is a domain admin & a member of a group with the Exchange Full Admin role.

Via Exchange System Manager, I can see that my account has the following permissions:
"Public Folders":
 - Full control via Exchange Full Admin group

"Outlook Security Settings" folder:
- Client Permissions : my account is listed as Owner
- Directory Rights: full control for Exchange Full Admin group
- Administrative rights: all permissions set to allow for my account name

I do not have the same problem in the only other folder in the Public Folders list, and am stumped as to what to do next.
Please help!