OWA Authentication

Hello together. I deployed the 2013 iApp for Exchange. I deployed the template like 20 times everytime it was working fine. Now i have a customer who gets, after entering credentials, a second login prompt. Which means I have to enter credentials twice to get into Outlook Web App.

 

When I login once and kill the session after, the login is still cached so at this time I just have to enter credential one time and he forwards me to OWA. Any Ideas? Something misconfigured on the other exchange site?