I started with enabling the domain-module enabled in ISPC3.
(via System>Interface config>Domains>Use the domain-module to add new domains)
Via System, Edit user, choose admin, and enable the module "domain".
You have to logout and login again. You will have an extra navigation icon (domains).
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.
Thanks to Germanius for his help (via DM).