This issue was caused by the expiration the of the SSL certificate. LetEncrypt, the provider of the certificate has revoked a certificate that older operating systems trusted. If you are still using Windows XP, now your only option would be to use Firefox version 50 or later, but I cannot say how long Firefox will continue to support XP. If you are using Mac OS 10.12.1 or earlier, then you are also affected. Firefox is the solution for you as well. For iPhone/iPad users, if you are using a version of iOS less than 10.0, you are also affected. If you have anything less than an iPhone 5, you cannot update the device's iOS version. Android users with less than version 7.1.1 are also affected.
If you continue to use the affected devices, then you will encounter this issue, not just with our site, but on the millions of other sites that use LetEncrypt for their security certificate provider.