Problem: When logging into the Exchange 2010 web app, I got the following errorâ€
â€œHTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfil the request.
Solution: The forms based authentication service wasnâ€™t started. Apparently failed upon a system reboot. Started the service and all was fine. Lesson: be sure to check all your Exchange services after a reboot.