You may encounter this error while performing actions like theme/plugins installation or WordPress version update which uses CURLSSL. This tutorial provides the description of this error with exact cause and its resolution.
RSS Error: WP HTTP Error: SSL certificate problem, verify that the CA cert is OK. Details: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed.
Various operations such as theme/plugin installation, WordPress version update require CURL. Now, Wordpress uses its own certificate bundle, overriding the global system certificate bundle. If you use the outdated certificate bundle of Wordpress, this error will appear. To fix this take follow steps.
Step 1 : Download the latest certificate bundle from this link.
Step 2 : Save it as ca-bundle.crt.
Step 3 : Upload this file on wordpress-installation-folder)/wp-includes/certificates/ via FTP or cPanel file manager. Try installing theme/plugin or updating WordPress setup.