OWA 2010 APM contacts list not loading

We have exchange 2010 OWA behind an internal LTM VIP and this all works fine when you connect to it internally.

 

If I make this available externally via APM (running on a separate pair of appliances in our DMZ pointing to the LTM VIP) using the portal access wizard - I can get connected and forms based SSO is all good, general mail is all fine.

 

The only thing that doesn't work is the contacts list. It just doesn't load up. Has anyone seen this before or know of any specific additional config I need to get this working?

 

Thanks in advance