I am having the same problem (I think):
When connecting to my DotNetNuke installation via localhost, everything is fine. However, if I connect from an external location (ie via http://www.xxxxxxx.xxx/dotnetnuke) I get an error.
I have checked my IIS settings and they look okay (identical to another site which I can connect to externally), so I was wondering if you can shed any light, or point me in the right direction in IIS/
Thank you in anticipation.