One of my clients still has Exchange 2010 infrastructure. Everything working pretty stable, but after one monthly updates and testing after system starting, I had problems with OWA. The problem may have been on at other levels, but I always test at the first place OWA and mail flow. Logon screen was fine but when you enter your credentials you receive the message:
After checking Exchange services (that is my first step in troubleshooting) status of the “Microsoft Exchange Forms Based Authentication” services was stopped. Simply starting service solved the problem. If the service is not started Outlook Web App will not authenticate users. Forms-based authentication enables a sign-in page for Outlook Web App that uses a cookie to store a user’s encrypted sign-in credentials in the Internet browser. If a session is inactive for too long, the server blocks access until the user re-authenticates.