The below error will be seen in the ssl_error_log typically located in /var/log/httpd for numerous reasons. Below is an explanation of what two of those reasons might cause Apache to not start and how to resolve the problem.
I recently reinstalled a CentOS 5.2 server running ISPConfig using apache for virtual web hosting. The first site I was reinstalling the RapidSSL certificate for was not working. It continued to tell me that it was a non trusted self signed certificate. Eventually I figured out the only issue was that ISPConfig was not restarting the web server thus not taking the RapidSSL cert. Once I restarted the web server manually everything worked properly. A couple things to note though are exactly how to install a SSL Certificate with ISPConfig and how to regenerate the servers self signed certificate if necessary.