In this issue, only three VPN clients can connect to ASA/PIX and connection for fourth client fails with the Secure VPN Connection terminated locally by the client. Reason 413: User Authentication failed error message. In most cases, this issue is related to simutaneous login under group policy. The ASA uses Cisco Adaptive Security Device Manager (ASDM) for configuration.
If the Inherit check box is not selected on ASDM, this parameter specifies the maximum number of simultaneous logins allowed for this user. The default value is 3. The minimum value is 0, which disables login and prevents user access.
In order to resolve this issue, increase the value for simultaneous logins.
Choose ASDM and then Configuration > VPN > Group policy. Choose Group > Edit > General and remove the inherit checkbox for Simultaneous Logins. Choose the appropriate value.
If you try more than three client connections again, it should work now.