The cURL error 35: failed to receive handshake, SSL/TLS connection failed can appear in the Really Simple Security debug log when the CURL function cannot connect to your website using SSL. cURL often uses a different set of certificates, shipped with PHP.
There are several things that can cause this exact error. In most cases, updating both cURL and PHP to a more recent version will resolve this issue.
If this doesn’t resolve the problem, please contact your webhosting company, as they can help you check why CURL is having issues.