I came across the following issue while planning an Exchange 2010 to Exchange 2013 upgrade for one of my customers.
They use OWA, and publish access to this externally using Cisco ASA clientless VPN. However, after some initial testing and investigation, we found that there is bug with the Cisco ASA Clientless VPN which prevents Exchange 2013 OWA working – bug CSCul27869 to be specific. Currently there is no workaround!
There is an enhancement request with Cisco to add support for OWA 2013. When this will be added is unclear. More details can be found here: