The token supplied to the function is invalid.The problem is SSL being required on the Exadmin folder in the Default Web Site used for Exchange management. This is only a problem when a third party SSL cert is installed because the internal name does not match the external DNS name used on the cert. In addition to disabling SSL for the Exadmin folder, I also had to use ADSI edit to remove the secure port in the Exadmin object in Active Directory. Now life is all good.
ID no: 80090308 Exchange System Manager
Detailed steps are at the bottom of the page here: http://hellomate.typepad.com/exchange/2004/04/public_folder_e.html