Active Directory Samba sharing issues

We have just implemented a Active Directory server for Authentication and storage. Each user has their own personal storage as well as group shares. These are working fine. The users can access the groups shares without needing to enter user credentials.

The problem arises when trying to access group shares that the user doesn't have automatic access to. Ideally we would want a username/password requester to appear so alternate credentials can be entered to access the share. An example is a tutorial room Mac. This has a generic Active Directory account with no group share access. We want the users to be able to browse to their group share and enter their own user details to access the share. Instead we get the following error:

A volume failed to mount.
The volume "share" could not be mounted.

Sorry, the operation could not be completed because an unexpected error occured.
(error code -6602)

The following error appears in the system log:

/System/Library/CoreServices/NetAuthAgent.app/Contents/MacOS/NetAuthAgent[2221] smb_mount: open session failed!: syserr = Broken pipe


I have tested this under both 10.5.6.and 10.4.11 and can confirm that it works okay under 10.4. This problem only seems to be with 10.5.