Hard to say from this. You will have to investigate further, with the use of logs. Here are some brief instructions on what to do to enable this.
On the VPN Client:
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On the VPN client, enable the Log Viewer. Click the Filter button and rightclick on IKE, IPSEC and AUTH one at a time and increase the log level to high.
On the VPN Concentrator
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Using URL: http://www.cisco.com/univercd/cc/td/doc/product/vpn/vpn3000/rel3_5_1/config/events.htm
INCREASE the "severity to log" of IKE,IKEDBG,AUTH and AUTHDBG event classesto "1-13"