OWA iApp Exchange 2016 - AutoDiscover not working

Hey since I deployed OWA iApp via F5 for a customer they claim that AutoDiscover is not working anymore. How can that be? OWA was running before on a TMG and now on F5. Since the change of IP/DNS to make it productive, the customers claiming that AutoDiscover is not working. My Access Policiy is like this:

Start --> Logon Page --> AD Auth --> SSO Credential Mapping

The single configuration parameters are kept basic (like template). Maybe it has something to do with "Cross-Domain Support"? or "Split Username from domain"?

Thanks! Would be nice of somebody could help me, this case is pretty urgent..