Does anyone know how to fix it?
The requested URL /checkout/onepage/ was not found on this server.
Apache/2.4.7 (Ubuntu) Server at idfr.com Port 443
Since you're using httpS but your cart link is http, so i can't even add products to cart if i don't switch to http. When i'm in checkout/cart (switched), clicking checkout will transfer me to checkout page which is default set httpS, then CORS happens again.
You need to check you secure and unsecure url.
As i checked, your server is not working well on secure link, so you need to config it first.
You can talk with your host provider about this problem. They will fix it for you. This is about server host, not about magento.