NLB works fine ... until AC Cluster creation

Hello !

I've setup a NLB Cluster with 2 server (W2k3, SP1, 2 NICs,).
I configured the Cluster with the NLB Manager for multicast operation.
It works fine, I can reach the Web Sites (displaying 
WshNetwork.ComputerName) with both, dedicated and virtual IP adresses.
When I create a AC Cluster (with keeping NLB configuration), i get a "IP 
address conflict" message at the controler and there is no (HTTP) response

from the dedicated IP address of the second Cluster member. The Cluster 
controler answers correctly to GET requests either with the dedicated IP 
address and with the VIP but when NLB switches to the second member I get
no 
answer.
What setting is changed during AC Cluster creation ?

C:\>ac loadbalance /status:all
Application Center 2000 Command-Line Utility.  (C) Copyright 2000
Microsoft 
Corp
..

Load balancing Settings for DExxxxxxxx10:
    Load balancing: enabled
    Status: online
    Weight: equal
Load balancing Settings for DExxxxxxxx11:
    Load balancing: enabled
    Status: online
    Weight: equal
Command completed successfully.

Best regards
Bernd