I solved it!!!
I had to uncomment
and add "www" in front of "ServerName" directive on both vhosts configuration files.
Should that have been done automatically by the ISPconfig system?
Off-topic: I found that i had to add
Alias /error/ "/srv/www/cywn.gr/web/error/"
on every vhost configuration file for the error pages to displayed correctly.
Hope these info helps you make ISPconfig better.