Is the IP of the subdomain of this website is identical with the IP address that you selcted in the website settings?
It's working fine without the creation of a site from ISPconfig. (just editing apache manually).
Thats not a good idea as this can cause conflicting websites which may casuse some or all websites to not work on your server. So do not add vhosts manually. You should eiser use ispconfig to create the websites or uninstall ispconfig if you want to manage the vhosts on your server manually, but do not combine manually management together with a controlpanel.