Yep, agree with Gdkitty. The fact that you can access one drive or folder indicates that the VPN is functioning, that your user credentials for logging via VPN are valid, and that the path and access permissions to the one folder, also via VPN, are valid. The question is what is wrong with the name, path, and/or access permissions to the other folder when you attempt to access it via VPN. Hopefully Gdkitty can chime in on this. I would expect that the access permissions for your account and shared folders might be, or are different, depending on where you logging in from, that is from an office at your workplace, or from a remote location via VPN. So when you ask your IT staff to confirm the access path and permissions, they would have to check both permission files, if that is how they are stored, and specifically check the file/folder/directory paths and permissions for external access via VPN. If there has been a change in any of those, the changes should be invisible to you as a user, but the problem is, if there was a change, it also has to be reflected in the VPN data as well.
I wouldn't change the CGN3 for the 2 model until you have absolutely nailed down the details with your IT staff and they have tested and confirmed access to the folder in question via an external VPN. That requires some leg work on their part.
You could also run a test with a neighbour. Talk nicely to him or her and promise a cookie or two to run a test with your laptop and VPN connection through their router. You would just have to ensure that the VPN pass-through is enabled on their router. That would proabably be easier than taking the CGN3 back and exchanging it. And it would confirm the access issue for that particular drive if you ended up with the same result.
I think that the problem with the drive access and picking up the CGN3 is a coincidence.