error trying to change reading options

i’m trying to change how many posts to display per page, but i keep getting the error

Forbidden
You don’t have permission to access /wp-admin/options.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

this only happens when i’m trying to change the “reading” options; the “general”, “writing”, “discussion”, etc. options work just fine. what can i do to fix this? TIA!