cURL error 60: SSL certificate problem: certificate has expired
If you are experiencing a cURL error 60: SSL certificate problem: certificate has expired error when testing HTTP Loopbacks (WordPress Admin Area > Product Sets > Tools > Test Loopbacks), here are 2 methods to handle this issue.
1. Contact your web host and ask them to ensure that your SSL certificate is not expired and to update it if it is.
2. If your SSL certificate is valid but you are still getting this error, please follow the instructions in this guide.
In brief, WP core has a file of root certificates /wp-includes/certificates/ca-bundle.crt which is used to check SSL of all requests created through HTTP API. In this file one of the root certificates has expired which was used to create a certificate for your site. Therefore, the request can not pass verification and generates this error.