Do you have PayPal express checkout on your website? Does it give the following error message when pressing the “Check out with PayPal” button?:

Unable to communicate with the PayPal gateway.

If you have tried everything and you are still searching for a solution, then you are on the right place. I’ve spent the last couple of hours to figure this out. If you enable the Magento logs, then you will find in your exception.log two errors:

exception ‘Mage_Core_Exception’ with message ‘Unable to communicate with the PayPal gateway.’

and

exception ‘Exception’ with message ‘PayPal NVP CURL connection error #77: Problem with the SSL CA cert (path? access rights?)’

What does this mean? It means that something is not configured well.  And this something is the parameter named CALLBACK. On PayPal developer’s page it is written clearly:

CALLBACK: (Optional) URL to which the callback request from PayPal is sent. It must start with HTTPS for production integration. It can start with HTTPS or HTTP for sandbox testing.

Character length and limitations: 1024 single-byte characters.

This field is available since version 53.0.

Most probably you are trying to use express checkout from a http shopping cart, which is not permitted by PayPal in production environments. Solution? Buy a SSL certificate (if you don’t have one already), configure Magento’s base secure URL in system configuration and it will work. Otherwise you won’t be able to use PayPal Express checkout.