You can re-generate the ISPConfig SSL certificate like this:

openssl genrsa -des3 -passout pass:yourpassword -out /root/ispconfig/httpd/conf/ssl.key/server.key2 1024

openssl req -new -passin pass:yourpassword -passout pass:yourpassword -key /root/ispconfig/httpd/conf/ssl.key/server.key2 -out /root/ispconfig/httpd/conf/ssl.csr/server.csr -days 365

openssl req -x509 -passin pass:yourpassword -passout pass:yourpassword -key /root/ispconfig/httpd/conf/ssl.key/server.key2 -in /root/ispconfig/httpd/conf/ssl.csr/server.csr -out /root/ispconfig/httpd/conf/ssl.crt/server.crt -days 365

openssl rsa -passin pass:yourpassword -in /root/ispconfig/httpd/conf/ssl.key/server.key2 -out /root/ispconfig/httpd/conf/ssl.key/server.key

chmod 400 /root/ispconfig/httpd/conf/ssl.key/server.key

Then restart ISPConfig:

/etc/init.d/ispconfig_server restart