If a server fails due to a ssl certificate problem, then follow these steps to resolve that:
1) Delete the symlink of the website where you enabled the ssl certificate in the sites-enabled directory.
2) Start apache
3) Login to ispconfig and idsable the ssl checkbox for the site and click save.
Then you can create new ssl certificates in ispconfig. Almost all errors related to ssl certificate creation are caused by using any special chars in the ssl fields as openssl is picky about that and will not create a ssl cert then. Better use only characters a-z and 0-9.