wcf service hosted in IIS6 with aspnet compatibility return 4001 but tracing as authenticated

[CS ADNT]

Hello,

I have a wcf service call using basicHttpBinding , security is transport
only with windows.
The asp.net web application hosting the service is using windows
authentication and allows only authenticated users.
I trace the http pipeline of the service call and after postAuthenticate, it
traces as authenticated, but the service call returns 401.

??
Thanks for help.
CS