When signing into OWA on our new Exchange environment, internal access works (on both CAS01.domain.com/owa and email.domain.com/owa) and the account can access the ECP/OWA without a problem.
When signing in from outside the organization, we can get to the login screen for OWA, but after entering credentials only a blank white screen appears.
Relevant versions of software are Exchange 2013 SP1 running on Server 2012 R2.
The environment has undergone relatively few modifications from the initial installation configuration: internal and external URLs have been configured for relevant VDirs (OWA, ECP, EWS, etc) and a third party SSL certificate has been imported and assigned to SMTP, POP, IMAP and IIS on the CAS, and SMTP on the mailbox server.