curl: (51) Unable to communicate securely with peer

This message often refers to an issue with the SSL Certificate that is setup for the domain. The error message is generally followed by a ":" disclosing additional information about the error. For example, the most common scenario would be the following:

curl: (51) Unable to communicate securely with peer: requested domain name does not match the server's certificate.

In this case, the description explicitly explains the SSL setup issue, which is a result of a mismatch. To gain a better understanding of the issue, you can run the cURL command followed by the "v" parameter for additional verbosity of how the request is being established:

[ Thu Jul 02 01:23:47 ~ ] $ curl -ILv cpanel.net
[...]
* Server certificate:
* subject: OU=Domain Control Validated; OU=PositiveSSL Wildcard; CN=*.cpanel.net
* start date: Dec 22 00:00:00 2018 GMT
* expire date: Jan 20 23:59:59 2021 GMT
* subjectAltName: host "cpanel.net" matched cert's "cpanel.net"
* issuer: C=GB; ST=Greater Manchester; L=Salford; O=COMODO CA Limited; CN=COMODO RSA Domain Validation Secure Server CA
* SSL certificate verify ok.
[...]

The above output is a portion of the result when the command is executed. Here, we can see in the SSL Certificate section of the output for the SSL Certificate setup. If this does appear to be the case, you'll want to look into the SSL Certificate to ensure its validity and possibly be reissued through AutoSSL.