There is a Windows trick that might work for you. Simply create a local account on the client machine which has the exact same username and password as the account that the server will use to access the client.
Try creating "ArcGIS Server Account" on the client with the same password that you used on the server.
If you want to share file based data then I think that the way to do it is to use UNC, e.g. \\machinename\Share, paths. Everyone has to use the same UNC path and it is this path that must be registered with the server.
If that doesn't work then I'm out of suggestions:(