Here are a few known causes.
Follow the instructions in the installation wizard to set up database and create your start node.
Please leave this file here to start ASP.NET when requesting the root folder (/).
If you know you have installed you might need to decorate the content class for the start page node with the [Template] attribute.
[Template("~/Path/To/Template.aspx")]
public void MyStartPage : ContentItem
{
}
This could happen when the database contains nodes that have no corresponding class. When using medium trust this could be due to an assmebly not beeing defined.