I have a simple Windows XP VPN set up, which works fine when I am using Broadband.
However if I use a 3G Vodafone USB Modem on the same PC connecting to the VPN times out when 'Verifying Username and password', although I can ping the IP address, connect to the internet and RDP to our office servers.
If I switch off the ZoneAlarm firewall and reload it the VPN works, but as soon as I switch the firewall back on the VPN disconnects. All the modem software is included in the list of programs to allow through (and it works using Broadband).
I cannot see any logs of what might be the problem.