When you use a single server in your organization, but wish to publish Free/Busy for Exchange users from another organization (using the Add-AvailabilityAddressSpace cmdlet and InterOrg replication or third party solutions), it is important to remember the following:

DAV requests made by the Availability Service to the Schedule+ Free/Busy Public Folder in your organization are sent using HTTP, not HTTPS, to the /public virtual directory. 
By default, when you install the Client Access Server role on a computer, the virtual directories are configured to require SSL.   In a single server environment, this will prevent the Availability Service from retrieving the Free/Busy information from Public Folders.

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