During the co-existence period of a transition from Exchange Server 2003 to Exchange Server 2010 you may encounter an issue with the legacy Outlook Web Access URL redirection.
When users connect to the Exchange 2010 Client Access server for OWA login they receive a second login prompt for the legacy URL.
No matter which credentials are entered into this authentication dialog box the login is not successful, and a HTTP 500 error is displayed.
The solution is to enable forms-based authentication on the Exchange 2003 front-end server. This is located in the Properties of the Exchange Virtual Server.
In the Settings tab enable forms-based authentication and click OK to apply the change.
Exchange will warn you that SSL must be configured and IIS restarted if you are not offloading SSL elsewhere, or have not already configured it in IIS. Click OK to close the warning (and obviously if you have not already got SSL offloaded or configured then you should go ahead and do that).