ok, the reason why it doesn't allow for this to work properly is in the vhosts. To fix this follow as instructed:
1. open "/etc/apache2/sites-avaliable/default" with your favorite editor
2. under "<VirtualHost *:80>" and right above "ServerAdmin webmaster@localhost" add "ServerName localhost"
3. save and exit your text editor
4. restart apache by invoking the command "apache2ctl -k restart"
5. go to your ispconfig and create a site with your server's domain name ie. domain.tld, and like normally add the aliases either www. or *. and save
6. wait a few minutes (2-3 mins) and voila it should be all working. You can test this, by modifiying the index.html file which would be under /var/www/domain.tld/web/ and adding a line. You should be able to see that line no matter whether you type in www.domain.tld
Hope this helps.