If you ever have tried to transfer WordPress site from one host to another, you will know you’ll run into problems. I did all by the book, by a nice tutorial at this link:

http://blog.triphp.com/how-to/how-to-move-wordpress-to-a-new-server-or-host.html

Great tutorial by the way! But the once everything was done.. Every time I clicked on one of the menu items of the new site, I got an index.php error. Not able to show the file, cause of a permission error.. Probably spent about 3-4 hours trying to figure out what a heck was going on. Then found a simple solution! Log in to the admin and reinstall WordPress. All works like magic now!
So, if you ever get such a message:

Forbidden You don’t have permission to access index.php on this server on this server

After you’ve copied all the files from an old WordPress site to a new one, and can’t access some.. simply reinstall WordPress, or update it, from the backend!