If you have dynamic and static crypto maps configured on the same device, make sure that the dynamic crypto map has been defined with the same name as the static crypto map.
If you use a Linksys wireless Access Point (AP) router, you may encounter Cisco bug ID CSCdu36784.
Using Linksys Beta 1.37.9a-BEFW11S4 will resolve this problem.
If this Cisco VPN Client is going through a firewall to connect to the Internet, make sure that appropriate ports are opened on that firewall. If Network Address Translation (NAT) is used, it should be one-to-one NAT, unless you are using IPSec over User Datagram Protocol (UDP) or IPSec over the Transmission Control Protocol (TCP) feature.
To resolve this issue, try reinstalling the VPN Client or lowering the Maximum Transmission Unit (MTU) to 1400 on the VPN Client.
For more troubleshooting information, capture VPN debugs on the VPN server (PIX Firewall, router, or Cisco VPN Concentrator) and check for any error message.