500 – Internal Server Error when using OWA with /exchange

The 500 – Internal Server Error is displayed when you access Outlook Web Access fromhttps://CASServer/exchange while https://CASServer/owa works fine.

This happens when you have separate Exchange 2007 Mailbox and CAS servers. Ideally the request should be redirected to /owa but you get the 500 – Internal Server Error right after typing in your credentials in the forms login page.

The redirection is not working because ISAPI Extensions are not installed on the Mailbox Server. ISAPI extensions handle specific incoming requests to the IIS server. The extensions are loaded when they are first needed and kept in memory until the host process shuts down.

To fix this issue, install the ISAPI Extensions on the mailbox server. This can be done by using the following command:

ServerManagerCmd -i Web-ISAPI-Ext

When the install is complete, restart IIS.