“Add Category” doesn’t work…

I installed WP in my host and when I try to add a new category I got the following error:

Forbidden
You don’t have permission to access /prog/wp-admin/admin-ajax.php on this server.

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

Apache/1.3.37 Server at master-arts.net Port 80

I’ve used WP in other websites and I didn’t have problems. Can somebody help me?