First make sure that the path is accessible to the server at all by going their manually using your own account. I don't know anything about your set up where you are (major corporation, mom-and-pop shop, whatever), but if there are security blockages on the network this may be an impossibility to start with. As an example, where I am security does not allow servers to communicate with each other in some of our domains through certain protocols/ports. So if I attempted something like this it would fail automatically because of the firewall restrictions.
Once you're sure the connection can be made, verify that the user account applied to the website's application pool has permission to make the connection. By default this account is set to "Network Service" and it is a local account. This will deny the website such permission. You will need to set this account to domain level service account and make sure that account has permission to make the connection.