In Exchange 2010, if you applied patches or simply rebooted your system, you might be presented with an HTTP 500 Error after logging into OWA. As shown below, all seems to be working fine because the login page is shown:
Thereafter you get the error:
Running an IISRESET returns the same results.
If you open up services.msc and navigate down to the Exchange Services, you will notice that the Microsoft Exchange Form-Based Authentication Service is not started. Start it and you will be able to login to OWA.
Hope it helps.