Cannot have multiple users logged in at once on a single client computer

I am running OSX 10.6 Snow Leopard Server on a Mac Pro, and have two iMacs (27.5") as clients in my workgroup.

Computer A : Server
Computer B, C : Client machines

The problem is that when user1 is logged onto the server network drive (/Network/Servers/ComputerA/Users/~user1/) from a given client (say ComputerB), it is not possible to login another user (user2) onto the server network drive from client Computer B.

It IS possible however for user2 to log in from another client machine (ComputerC) without any problem and access his home directory.

Similarly it IS possible for a local user to log onto ComputerB while a network user is logged onto the central file server.

If user1 is logged into ComputerB, and user 2 is logged onto ComputerC, if user1 ssh's over to Computer B, he can log in, but he cannot access his home directory, getting the following error:

Could not chdir to home directory /Network/Servers/nplab0/Users/chris_dimattina: Permission denied
-bash: /Network/Servers/nplab0/Users/chris dimattina/.bashprofile: Permission denied

and gets sent to the root directory of ComputerB.

Any help would be appreciated!!

Thanks,
Chris