I've created a virtual server to run a new WP installation.
Before the this WP installation, I used to have an index.html which doesn't exist anymore.
After installation, the http://domain/wp-admin is working fine but when I try to access http://domain or http://domain/index.php, it tries to download a file with a weird auto-generated name with no extension. When I check this file on a text editor, it contains the html code that used to be on that old index.html file.
On the same root directory I've run a "phpinfo()" page and it works fine.
I've restarted Apache many times. To me, looks like some "cache" issue, but when I try to access from a different machine/mobile, I get the same wrong behavior (the weird download).