The key on the VPN 3000 Concentrator is not configured correctly. The user cannot access the concentrator to change it because all access to the concentrator is being authenticated with the TACACS+ server.
If you know what key was entered in the VPN Concentrator, you can reconfigure that object's key in the server to match.
If you don't know what key was entered in the VPN Concentrator or you do not want to reconfigure the server, then you will need to access the concentrator through a direct connection to the console port and change the key with the command-line interface. For more information, see Using the Command-Line Interface.