The Symptom
Outlook Web App (OWA) shows its sign-in page as normal, but after you enter your credentials it returns an HTTP 500 error or a blank page instead of opening the mailbox.
Running IISReset /NoForce, or even rebooting the server, doesn’t change anything.
The Cause and Fix
This is typically caused by the Microsoft Exchange Forms-Based Authentication service being stopped. Open Services on the Exchange server and start it. OWA works again immediately.
