Hey Reynen,

I had this problem initally as well.  I think that you need to make sure that you do two things.  Make sure you have that domain name pointed at your dnn install and second, after you create a portal you need to restart dnn before it recognises the new portal.  I did that by hitting the restart application button, under host settings.  Alternatively I read somewhere that you can also restart it by adding an enter to your current web.config file.

The workaround, at least the one that I've been using successfully, is to login to the newly created portal as the portal admin (eg - if newly created portal is portal2 with admin2 as the portal2 admin, login as admin2 on portal2 and make your changes).