Thats suggesting one or more SSL certificates you have applied have an issue which is why apache is prevented from starting.
Have you recently changed/altered ssl certs? Have any expired?
Google is your friend on private key errors.
This user dealt with it previously;
However, I'd try commenting out " SSLEngine on" in any vhost files in /etc/httpd/conf/sites-enabled and see if apache then starts. Suppose it depends if you have https just for ispconfig or you have client sites using it.
If you get it back up you can always reapply the certs through the interface.