Username's aren't authenticated to domain controller while using VPN

We have had a couple of users lately who use VPN often from their home.  Once connected to the VPN successfully (they can ping the server, get a virtual IP address from our network), they run a simple login script that maps a few network drives from the server.  The problem is, the batch file errors out when they try to run it...sometimes.  When it errors out, it complains of an invalid username.  When they simply try to manually connect to a shared resource on the server, by typing \\servername\share in the run command, it prompts them for a username and password.  They type in their username and password, and it gives them the following error:

"The username you typed is the same as the username you loged in with. That username has already been tried, the domain controller cannot be found to verify that username."

Yet, if they use the Administrator account, it then maps just fine.  Any idea what the problem is here?  Many times it works perfectly fine, but other times, they get this error (and this is really only happening to two users...the rest all work fine all the time).

We have a Sonicwall 2040 firewall and run a single Windows 2003 domain controller server.

Thanks.