Can’t change user password from command-k (AFP) – OSX server bound to AD

Hello, we have an OS X server, whose role via Open Directory is "Connected to a Domain" in AD. If the users try to change their AD password from the option in "connect to server..." (command-K) in the Finder (through AFP) they get an error message. If the user is created directly on the OSX server (therefore is not an AD user) they can change their password. I have been searching many forums trying to understand the reason but with no luck. Server and client are OSX 10.4.11 whereas the AD environment is Win2003. Many thanks for your kind attention. Ciao Carlo