When I try to make a change in Meta Tags Manager Meta Tags Manager:

Forbidden

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

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
How can I fix this?
thanks

I'm also getting this error when trying to enable JoomSEF:

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

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

Hi,
If it's saving JoomSEF's Configuration what causes this problem, try checking your web server's firewall settings. Sometimes it doesn't allow the default value for "Filter these words" option in JoomSEF's Configuration on the Advanced tab. The default value is:

which some firewalls don't like.