The FA uses Let's Encrypt certificate for the website SSL (https). That company found a bug and will be revoking certificates worldwide tomorrow. Some details here: https://arstechnica.com/information-tec ... ority-bug/
I will be monitoring the website to renew the certificate, and there is no loss of data, etc. However, depending on timing, you may get SSL errors about invalid certificates tomorrow and/or the next day (March 4-5). This is what that means. Traffic during this time will still be encrypted; it's the code that verifies that you are on the right website that it's warning about.
Just a heads up so people aren't surprised if they see these errors.