All of your existing websites have '*' selected in ISPC? Then you should add an IP-Address and switch the sites from * to the correct IP.
rudolfpietersma described it here:
I also did not have an IP address added in ISPC3.
(via System>Server IP addresses>Add new IP address)
Changed all website from * to the IP address.
If you changed the first website to a IP, all others will not work anymore. But don't worry, go on changing every site. All sites you changed will work.