This error message indicates incompatible preshare policy and that tunnel negotiation has failed. More information can be obtained at "http://www.cisco.com/warp/public/707/17.html".
Your old VPN session might be timing out because of the configured parameters. You might have defined a "lifetime" somewhere. If your session is timing out each time, after being active for the same period of time, configured timer values are certainly causing it.
