telnetPORT25 wrote a great article explaining the step-by-step process, along with screenshots, to fix this problem. I'm listing the high-level steps here (mainly to act as my long-term memory).
- Logon to the Exchange 2007/2010 CAS server
- Copy the %SystemRoot%\System32\inetsrv\iisadmpwd folder and files from the OWA 2003 FE server to the CAS server's %SystemRoot%\System32\inetsrv folder
- Open IIS Manager and add a new Virtual Directory off the Default Web Site named IISADMPWD with a physical path of %SystemRoot%\System32\inetsrv\iisadmpwd
- Right-click the new IISADMPWD virtual directory and select Convert to Application
- Select the MSExchangeOWAAppPool
- Restart IIS (iisreset /noforce or select the server in IIS Manager and click Restart)