Straight after upgrading my Exchange 2010 servers to Service Pack 3, the Outlook Web App page (OWA) published over Unified Access Gateway (UAG) started throwing the following error:

You do not have permissions to view this folder or page

UAG requires Basic Authentication over OWA.  For some reason Integrated Windows Authentication got turned on after the SP3 upgrade.

http://technet.microsoft.com/en-us/library/ee921443.aspx

Turning Integrated Windows Authentication off via the Client Access OWA settings resolved the issue.  Though beware, you have do this on all your Client Access servers.