If you have set a 403 custom error page and you have "deny from all" rules in your .htaccess file, access to the 403 custom error page will also be denied.

The easiest way to resolve this issue is to allow access only to the 403 custom error page.

So, if you have the following rule defining your 403 error page:

ErrorDocument 403 /~cpanel_username/403.html

you should add the following rule to your .htaccess file:

<Files 403.html>
allow from all
</Files>

in order to allow access to the 403 error page.