Apache should use SSLCertificateChainFile instead of SSLCACertificateFile
ISPconfig uses SSLCACertificateFile for SSL bundle certificates.
The Apache documentation prefers SSLCertificateChainFile:
http://httpd.apache.org/docs/2.4/mod/mod_ssl.html#sslcertificatechainfile
Is there a reason why SSLCACertificateFile is used?