Duplicate directive NameVirtualHost in ispconfig.conf and ports.conf?
In /etc/apache2/ports.conf and /etc/apache2/sites-available/ispconfig.conf I found the Apache directive NameVirtualHost *:80 and NameVirtualHost *:443, I think these are duplicates and cause the Apache warning below.
[warn] NameVirtualHost *:443 has no VirtualHosts [warn] NameVirtualHost *:80 has no VirtualHosts