When you fill in your credentials on the login screen of Outlook Web Access or OWA (2010) and click submit you receive an Internal Server Error 500 and the URL shows /owa/auth.owa
There are no events logged in Application or System log that explain this problem.
IISreset does not help.
The problem and solution could be very simple.
Check the the service “Microsoft Exchange Forms-Based Authentication service” is started. If not, start the service and try to login to OWA again. Problem should be solved.
If the service doesn’t start or fails frequently you’ll have to investigate that further.