HTTP Error 403 When Trying to Login

It sounds like you did not correctly move WordPress and your local install is trying to login to the hosted site. I am guessing that this is the problem becasue of this sentence, “Data may not be posted from offsite forms.” Your login form should not really ever be “offsite”. You need to change the site URL just as if you’ve moved the site from one domain name to another.

If it isn’t multisite, you can edit wp-config.php like this:

define('WP_HOME','http://example.com');
define('WP_SITEURL','http://example.com');

“http://example.com” in your case will be something like “http://localhost/yoursite” or an IP like “http://192.168.1.2/yoursite”. The IP is better because it will allow the site to work for any computer on the local network. You may also need a port number after the domain, like “http://192.168.1.2:8080”. You will have to use the actual values for your local server. MAMP should be able to tell you what those need to be.