Issue:  User is unable to complete the checkout process or unable to log in. 

What's happening: Most likely, the user/s in question are using an older version of a browser which supports non-compliant, insecure SSL protocols to communicate with the server. 

Solution: Once these users upgrade their browsers they should not have this problem. Mitigating actions on the users' side differ depending on the OS/browser version they are using. However, the general rule is that upgrading their OS to the latest version available and their browser to the latest version available should solve the problem.

Note: Older versions of an OS don't always support the latest version of a certain browser. This is why it may be necessary to update the OS as well as the broswer.

The main culprit is TLS v 1.0. This protocol was found to have a vulnerability and thus cannot be used to comminicate with areas of the site that require a secure connection (such as ecommerce). If your site is set up to have an entirely secure domain, it will stop users from logging in completely. 

If a user is unable to upgrade their OS/browser any further, they can attempt to deactivate TLS v 1.0 in their browser's settings: