I have faced this issue in webmin/virtualmin server where Let's encrypt SSL was installed on the domain and while installing jetpack in WordPress it was giving error.
The problem is with SSL of the domain. If you are using let's encrypt SSL on the domain then it might give you error. It's because SSL pem authentication file is missing.
How to fix it?
To fix the issue what you have to do is download the pem file and install it on the server.
Replace the ssl.ca file with this file:
It is the correct CA intermediate certificate for newly issued Let's Encrypt certificates.
This is the old one, which you are currently using: