The 'Access-Control-Allow-Origin' header has a value that is not equal to the supplied origin

I am trying to login with ajax to an API and I get this error:

XMLHttpRequest cannot load. The 'Access-Control-Allow-Origin' header has a value that is not equal to the supplied origin. Origin 'http://localhost' is therefore not allowed access.

I read all about this error, all over the internet, and I've tried all the solutions I could find online. I modified the .htaccess and apache httpd configuration file according to the CORS instructions here: http://enable-cors.org/server_apache.html

Access-Control-Allow-Origin: *

Nothing seems to be working. I'd really appreciate if you guys can help me out with this. Thank you!