Cisco VPN error: Secure VPN connection terminated locally by the Client. Reason 412: The remote peer is no longer responding

This error occurs for the following reasons:

• The user is behind a firewall that is blocking ports UDP 4500/500 and/or ESP.
• The VPN client is using connecting on TCP and the default TCP port 10000 for NATT is blocked.
• The internet connection is not stable and some packets are not reaching the VPN concentrator/server or the replies from the server/concentrator aren’t getting to the client, hence the client thinks the server is no longer available.
• The VPN client is behind a NAT device and the VPN Server doesn’t have NAT-T enabled. In this case the user will not be able to send or receive traffic at all. It will be able to connect but that’s all. After some time the software client deletes the VPN tunnel.