You don't have permission to access /administrator/index.php on this server

Just in case you happen to get this error...

Forbidden
You don't have permission to access /administrator/index2.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Solution is to edit your .htaccess file by adding this lines.

IfModule mod_security.c
SecFilterEngine Off
SecFilterScanPOST Off
/IfModule

I can't put < > on the above lines so just add it by yourself. If you don't know how, just give a comment and get back to you asap.