Impersonation between a web app and a web service on same iis fails

Hello everybody

Big problem here. I admit, have no further idea and also our networking buff with 20+ years experience scratches his head. But perhaps it is something simple.

Scenario:
IIS 6.0 Win 2K3 (all patches applied, automatic update)with multiple websites configured.

WebSites
  Default Web Site
  Intranet
  www.xy.tld
  ....

I install my ASP.NET 2.0 application under default website like

WebSites
  Default Web Site
    app
      clientapp
        webserviceapp

"clientapp" has in config file impersonation = true and authentication = windows
"webserviceapp" has in config file authentication = windows

This works. My application works fine and authe ...