This is from the vpn 3000 concentrator ver 4.1.7 doc:
Web VPN uses global authentication and authorization settings, not the settings configured for the group. The first active server, independent of type, is used for authentication and authorization of WebVPN sessions.
I think that your issue may lie with your global authen server settings. If you have a global radius server defined, make sure it is 1st in the list; if none are defined then add it and make it the 1st.
Other vpn parms, where the type of server is checked, so that if you have an already configued group that uses internal or SDI and you have global authen servers of different types - only that type of server is queried if there are no group specific server types configured. WebVPN will use any server.
Let me know if this helps.