If you are using outdated certificate bundle then below error will appear.
To resolve this issue you can follow below steps:
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.
- You have to download the latest bundle certificate from internet.
- After that save this .crt file.
- Upload this downloaded file in /wp-includes/certificates with File manager or FTP account. Now try to install theme or plugin.