QuickBook Pro 2007 Error Codes (-6000,-82) When Using Over VPN

I have a client using QuickBooks 2007 Pro with six licenses (five concurent users).  The data file is installed on a Windows 2003 Server and all the clients are running Windows XP Pro.  One of the users is located in another state and accesses the network via a VPN with SonicWall TZ 170 firewalls on both ends.

For several weeks, the user on the VPN was able to open the QuickBooks file (via a Q:/ network drive mapping) without a problem and use it normally.  About a week ago, she began to get the following error when she tries to open the file: "An error occurred when QuickBooks tried to access the company file" with codes: (-6000,-82).

None of the users at the main office are having a problem access this company file and if I copy it to the remote users's local drive, QuickBooks can open it just fine.  So, I'm thinking it's a VPN issue.  But, I'm not aware of anything that has changed on the network during the time this stopped working.  I went through troubleshooting with QuickBooks tech support and, besides trying to sell me a lot of stuff, the troubleshooting steps they suggested didn't find anything.  Has anyone else here dealt with these error codes when using QB over a VPN and have any suggestions?