To enable SSL I didn't use the ispconfig ssl tab because I already had the files on old server.
So I added to ispconfig.vhost the following lines:
Ok, thats wrong and can not work. Please undo the changes you made manually.
Then enable ssl in the wesbite that shall have ssl access and create a new self signed certificate in ispconfig for that site. Test if the ssl cert works. Afterwards replace the .key and .cert file in the ssl folder of the site (folder /var/www/yordomain.tld/ssl/) with the key and ssl cert that you already have and restart apache. Dont change any filenames, just replace the key and cert data in the files.