Sharing&security issue

Office machine with XP has a folder that has data that needs to be accessed,
copied and replaced. Machine is connected to DSL thru' snapgear pro hardware
vpn router. User and password are set in the router and the protocol is
MSCHAP2/pptp. Permission and security are set thru XP(simple file sharing
disabled).The overseas machine is able to map the folder,access data, copy
files and replace them( I think they run 98&2000). Another XP machine from
another site with the same settings(username, password and protocol) as the
overseas machine is able to connect to the router but "access denied" when
trying to map the same folder. The machine that is unable to map as needed
is able to map a drive on another office machine running XP that the user
and password should not have access to. Any ideas on what is going on?