Is this on SBS 2003 and is it Exchange Server 2003? I found this link with the same problem but it is on a clustered exchange server setup. Hopefully this information regarding security rights on exchange server 2003 SP1 will help you try to find the problem:
An internal server error is pretty basic, so this above mentioned article may not apply, we’ll need more information to help fully.
Try going into the browser options (Tools/Internet options) and go to the advanced tab and turn <b>off </b><i>Friendly HTTP Error messages</i>, that will give you a better error we can work with to help.