Passthrough login not working

Hello

I am having problems launching applications from an exteNd Director
Standard 4.1 portal. Connections to the Citrix server are established,
but we always get a popup saying the the login to Windows failed.

Citrix server is Windows 2000 SP4, Citrix MetaFrame XP FR2, Client32
4.90 SP2, ZfD agent 4.0.1.

DLU policies are in place and dynamic user accounts are being created if
logging on to the server through an RDP connection, so we know that DLU
works.

I have unchecked the Always Prompt for Password option in RDP connection.

ZENTCAS service is running.

Any ideas would be greatl appreciated.

Jacques