Site's name: "Blog" and I've added an empty default.aspx page in the same folder and have also mapped that in the administration interface.
You must assign the folder name to the site but there should not be any url mapping. If you created any url mapping for default.aspx you should remove that as it would probably cause problems.
Next, as with any error, the first step is basic troubleshooting to find out the error details.
Hope that helps,