Users may receive the following error when access /EXCHANGE or /PUBLIC on an SBS 2008 server. Users can still access OWA using the /OWA path. Note: SBS 2008 users should always use /OWA or RWW to access OWA. This issue is a very rare issue, but if encountered can take a very long time to troubleshoot.
This issue can occur if the HKLM\SYSTEM\CurrentControlSet\Services\InetInfo\Parameters registry key is missing. To resolve this issue recreate the key and restart IIS.
Source:→ SBS Blog