It works now ! I found out the problem:

In the DNN-Portal Aliases I have to change the position: 1.) localhost 2.) www.geopic.at AND I have forgotten to remove the "Application Name" within the IIS.

Summary for changing a DNN-installation from a v-Dir (WinXP - developer machine) to a production environment (Windows Server 2003) without a v-Dir (real web-site with it's own app.pool):

Imagine that on the server (production) your DNN-portal is perfectly running on www.yourdomain.com with own application pool under IIS 6.0

1. check your Portal Aliases (also the position): 1.) localhost 2.) www.yourdomain.com
2. copy the whole directory from your server to your developer machine (e.g.: c:\inetpub\yourportal)

You have now the exact copy of your server on your dev. machine - next steps are:

1. Open IIS
2. Open Properties for "Default Web Site"
3. Change to Tab "Home Directory"
4. change your local path to "c:\inetpub\yourportal")
5. Don't remove "Application Name" (should be "Default Application")
6. Within IIS (within "Default Web Site" - tree structur) you have a tree-node: yourportal 
7. Check the settings of this node:
8. Local Path: c:\inetpub\yourportal - AND: remove the Application name (all within the Tab "Virtual Directory")

THAT's it  - hope I could help someone

Cheers,
Robert