Problem with Offline Files Sync

I'm the IT manager for a 30-person shop. We use Folder Redirection for
Desktop and My Documents folders for all users. For laptop users, I've also
configured Offline Files on those shares. All configuration is done via
Group Policy, for ease and consistency. The setup has worked well over the
years, but I've observed a problem on a few of our laptops that I could
never figure out, and now it's happening to me too.

File synchronization normally takes about a minute for the number of files
that I have. Starting sometime in the last 1-2 weeks, during sync, my
laptop suddenly goes offline, saying "Offline Files (\\server\share on
domain): Unable to connect to '\\server\share.' The specified network name
is no longer available." My connection to the network is rock solid,
however, as is the server's.

I found the following KB article: http://support.microsoft.com/kb/231468.
Although it refers to Windows 2000 and I'm running XP SP2, I played a hunch
and, while still connected via ethernet, enabled my Wi-Fi card and also
connected to our local Wireless LAN. Synchronization succeeded. I then
disabled the wlan card, and the next sync failed.

I have a half-formed theory, but am not sure where to go from here. Here's
more information, followed by what I think I know:

- We run a SBS 2003 box. Users who are out of the office use the SBS VPN
client to connect to our office to access local resources.
- Sometimes that VPN client operates over a Wi-Fi connection, either at a
public hotspot or at their home, and sometimes over a cellphone laptop card
(Alltel Wireless).

- The problem may occur only to people who have performed an Offline Files
sync across the VPN connection running over some connection other than our
standard LAN connection. I need to do further testing to verify this, but
that is my belief at this point.
- I did not have this problem until a recent offsite meeting, where I
attempted to use the VPN connection at various times over both a Wi-Fi
connection, and a cellphone laptop card. The connection would not establish
via that particular Wi-Fi connection, but the laptop card worked fine. My
problems appeared to have started after that meeting.

Thoughts? Suggestions? This problem affects 4 or 5 people in my office -
I'd love to finally fix this thing.

Thanks in advance!

Bryan