I've this problem, I think is a XP problem, but I don't understand where is, because it's a new pc with only few softwares (like ms office, openoffice, skype).
If I connect to my vpn concentrator via webvpn all works properly, if I connect with client, I insert user/pwd and connection seems ok, but no traffic pass.
The profile is the same that webvpn.
I've also tried some other vpn to other concentrator and problem is the same.
I've tried some versions of the client (from 4.x to 5.x).