After upgrading from 4.5.X, I navigate to installer.php and get this error:
Cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to 'PHPfastcgi'_
I delete the line out of web.config (even though it was in previous version just like that) and then I get this error:
The page you are requesting cannot be served because of the extension configuration. If the page is a script, add a handler. If the file should be downloaded, add a MIME map.