I keep finding that on our small company LAN (7 users, 3 servers) that some servers keep becoming "not accessible" for the purposes of file sharing. They display the message "\SERVER is not accessible. You might not have permission to use this network resource. The user name could not be found". But I don't know why "the user name could not be found" as all the machines are on the same domain and the PDC and BDC seem to be behaving OK.
VPN seems to be the cause:
It turns out I can see the server if I use the IP address (\\18.104.22.168\ etc) or the FQ active directory name (eg \server.domainname.local) but not if I use the server name on its own or a mapped network drive originally created from the "short" name. Oddly though, my machine has no issue resolving the server's DNS name as I can ping the machine name OK and it immediately comes back with the IP, however nslookup seems to fail.
It seems to be a problem with how Windows looks up machine names when connected to VPNs. When I'm connected to a VPN, windows seems to use the DNS assocated with the VPN and not the one on the domain controller. This behavior to me, seems incorrect as surely that would mean connecting to any VPN would break any ability to lookup local machine names for servers and printers etc. So I guess the real question now is, how can I make my machine still search the local Active Directory DNS (the PDC) even when connected to a VPN?
More info in my comments below.