Microsoft Exchange 2013 iApp - Can't login to OWA or ECP if more than one server is active in pool

I was experiencing this issue as well.

My External Domain is foo.com. My internal domain is corp.foo.com. The iApp was used to setup OWA/ECP, etc. for owa.foo.com. A wildcard cert for *.foo.com for Client and Server SSL on the F5 (configured in the iApp). Cert was also imported into exchange and applied to the IIS role for each CAS. I had to set all my Exchange URLs to match "owa.foo.com" using the PowerShell script at http://jaworskiblog.com/2013/04/13/setting-internal-and-external-urls-in-exchange-2013/, running it three times (once for each CAS server).

As soon as this was complete, I could login without issue. No cert errors, no issues.