It just so happened that we had to restart our Exchange-Server (Exchange 2010, SP2).
After the reboot was completed, everything seemed to work like it was supposed to. But when I checked the owa later that day I only got a blank page after login. What was happening?
After digging around the interwebs a little I found that there was a service not running when in fact it should be. It is set to start on boot, but somehow did not. Simply start the ‘microsoft exchange forms-based authentication service’ or in german ‘Formularbasierte Authentifizierung’.
After that, owa is up and running again 🙂