I've never tried to install DNN without domain names pointing to the server. ie I've never just tried to use the IP address. I'm *assuming* that DNN has some sort of smart ISAPI intercept that maps the aliases into DNN dynamically.
Of course since you can see the default portal and log into admin, this is probably a bit of a red herring!
I'd try assigning a doman name to the IP address AND mapping the domain name using the host header in IIS to DNN.
Is there any other error message in the Event Log?
Do you have visual studio on the dev server? you may want to try to find the code that does the intercept to figure out what the heck it's missing...
Cheers,
Lance