Synchronisation problems over a VPN

When I am in the office I use file synchronisation between Windows 2000 Server and Windows XP. I connect in to the office using a Watchguard Mobile User VPN client and connect to a firebox. When i am online over the VPN I am not able to work with or manipulate offline files.

An example: If I am in outlook whilst connected to our exchange server over a VPN I cannot attach an offline file to an email. To do this I have to close down the VPN connection, shut outlook, reopen outlook, work offline and connect the required file, then reconnect the vpn and send the email.

The error I get when I try to attach a file in this manner is: 'The system detected a possible attempt to compromise security. Please ensure that you can contact the server that authenticated you'

Just one final point when I try and contact an offline drive whilst being online I am prompted to login again. The only login that is successful is the network administrator password. My login which includes admin priviledges is not accepted?

Has anyone any ideas why this might happen?