System error 53 - The network path was not found

In our loftware Windows 2000 box we have mounted a NAS device as Z:. It stopped working a few days ago. Mapping it with net use z: \\nasmount1\loftware again gave this error:

System error 53 - The network path was not found.

Symptom: when using net view ip or computername, you get system error 53. Resolutions:

1) if it is domain environment, check your WINS;
2) if it is peer-to-peer workgroup, enable NetBIOS over TCP/IP;
3) make sure the machine is running;
4) make sure file and Printer Share enabled on remote computer;
5) make sure client for ms networks is enabled on local computer;
6) make sure you type the correct name.

However all of the above were ok. On top of that, it mounted fine if I tried the net use command on my laptop. After many hours of trial and error, I realized that I am working from home and connected through VPN. I pinged a few colleagues in office and asked them to try the net use command. They got the same error as the loftware box. It looked like firewall settings had been changed. I installed VPN client on the loftware box and connected through VPN. Voila, net use command worked fine. I have to now talk to the firewall guys to find out what changed and revert to previous state.