Hello,

There are several reasons, which can result in this error message. Some are mentioned in the VPN client release notes:

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If an attempt to load the VPN Client is made before the Clients Service loads, the following error occurs: "The necessary VPN sub-system is not available. You will not be able to make a connection to the remote IPSec server."

Workaround:

Wait until the Service has loaded, then start the VPN Client.

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When installing Release 3.5.3 and later, up to Release 3.6.3 of the VPN Client on a Dell C810 with the integrated Truemobile card, the following error occurs:

Installation error. Windows has encountered errors while trying to run DNE.exe.

Program will be closed.

The VPN Client still appears to install correctly, but when starting the application, the following error reads:

The necessary VPN sub-system is not available. You cannot connect to the remote VPN server.

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The message, "The necessary VPN sub-system is not available. You will not be able to make a connection to the remote IPSec server." might appear on a PC when Start Before Logon is enabled on the Client and ZoneAlarm is also running. The message appears when the ctrl+alt+del key combination is pressed. This has happened because the Cisco Systems VPN Service has terminated unexpectedly.

Workaround:

Logon to the PC with cached credentials, open "Services" in control panel and start the VPN service. A connection to the VPN Concentrator will be possible once the service has started.

Another reason could be that someone disabled the Deterministic Network Enhancer in the Network settings of the NIC.

It would be helpful to know the version of your client in order to narrow the search.

Hope this helps! Please rate all posts.

Regards, Martin