I will start with a little background information before asking my question.
5-site MPLS w/ cloud firewall. SSLVPN client connects to Cloud Firewall and authenticates users against our PDC via LDAP and are given an IP that is routable on the MPLS.
Virtualized Server 2003 SE as PDC @site1
Virtualized Server 2012 as DC Slave @site5
Virtualized Server 2003 SE as File Server @site5
XP Pro client connecting into MPLS from home using the SSLVPN.
An employee has recently started working from home. She has taken her company-owned domain-connected laptop home to work over our SSLVPN connection. The user is able to login to the SSLVPN using domain credentials and is able to communicate with the three servers above. The user has 2 shares that they need to access on the File Server. The shares have 4 groups assigned in permissions/security: Accounting, local admins, Domain Admins, authenticated users. While they are at site1 they are able to access the shares with their credentials. When they connect from their house via SSLVPN and they try to access those shares from their domain account they get the message "unable to locate DC to authenticate this user" (their username is in the Accounting group). However, I am able to enter my credentials (Domain Admin group) and it authenticates and allows access just fine.
I have reviewed all my share and security settings and cannot find anything to cause an issue there. I have also edited her computer's host file to automatically resolve each server to the proper IP address as Servername and servername.domain.com. Verified communication via hostname and FQDN is possible via ping. I have looked into how XP locates a DC and I have been unable to determine why it is unable to locate a DC to authenticate against. Right now this user is having to access those share files via RDP to our application server at site5, but this is not optimal.
Why are my domain credentials able to be authenticated against the DC but the user's credentials illicit the "unable to find DC to authenticate user" when trying to access the shares.