Apache server, followed the guide from here: https://www.startssl.com/Support?v=21
SSLProtocol all -SSLv2 -SSLv3
Error code: SEC_ERROR_UNKNOWN_ISSUER
... You may need to install an Intermediate/chain certificate to link it to a trusted root certificate
The SSLCertificateChainFile option was obsoleted in Apache version 2.4.8 and any chain certificates need to be added to SSLCertificateFile instead. Since you are using 2.4.23 based on your comment this means that this setting was ignored. This means that no chain certificates were sent to the client, causing the validation error. You should have gotten a message in the error logs though pointing out the invalid setting.