Resuelto añadiendo unas líneas al fichero de configuración que ha quedado así:
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<staticContent>
<mimeMap fileExtension=".mp4" mimeType="video/mp4" />
<mimeMap fileExtension=".ogv" mimeType="video/ogv" />
<mimeMap fileExtension=".webm" mimeType="video/webm" />
<mimeMap fileExtension=".ogg" mimeType="audio/ogg" />
</staticContent>
<defaultDocument enabled="true">
<files>
<clear />
<add value="index.aspx" />
<add value="index.asp" />
<add value="index.htm" />
<add value="index.html" />
<add value="home.aspx" />
<add value="home.asp" />
<add value="home.htm" />
<add value="home.html" />
<add value="default.aspx" />
<add value="default.asp" />
<add value="default.htm" />
<add value="default.html" />
</files>
</defaultDocument>
<security>
<authentication>
<basicAuthentication enabled="true" />
</authentication>
</security>
<httpErrors errorMode="Custom">
<remove statusCode="404" subStatusCode="-1" />
<error statusCode="404" prefixLanguageFilePath="" path="/OaOInternal/DefaultWebs/sedoCurrent/Error404.aspx" responseMode="ExecuteURL" />
</httpErrors>
</system.webServer>
</configuration>
Fuente: [URL]http://www.webmasterworld.com/microsoft_asp_net/4138302.htm[/URL]