VPN logon problem

Hi,

Using Windows 2003 DC, NT4 BDC. Using 3rd party VPN client
(Sonicwall Global VPN client). The VPN tunnel appears to
be working perfectly. I can connect immediately and log
into network.
My problem is this:
I can only log in with one persons account. I have been
unable to log in as anyone else, including the
Administrator account. The user account that I have been
able to log in with is a member of the Admin groups. I
tried to add a different user to the Admin groups, but it
made no difference. Still didn't work. When I attempt to
log in with all other user accounts, I get an error that
either the user name or password is incorrect, but it
isn't the case.

Any help much appreciated, thanks.

Mark