The VPN 3000 Concentrator can reject users from logging into WebVPN for several reasons. This occurs due to the maximum allowed sessions for WebVPN (if more users are trying to connect then specified).
To resolve this issue, perform these steps:
Select Configuration > User Management > Base Group > General > Simultaneous Logins.
If you want more users to connect, increas the value. Decrease the value if you want to limit the number of users.