When using IPsec on Windows 10 and you see the error message "Error Code:800 DotRas.RasDialException: The remote connection was not made because the attempted VPN tunnels failed. The VPN server might be unreachable. If this connection is attempting to use an L2TP/IPsec tunnel, the security parameters required for IPsec negotiation might not be configured properly." appearing in the connection log of the VPN Manager, the following steps are suitable only to fix the issue if it has its cause locally on the computer. If it is caused by a faulty network connection or a blockade of the connection to the VPN server e.g. by a firewall, the issue can not be solved with this guide.
First exit the VPN Manger if you have not done this already. In order to execute the following commands you require a Command Prompt with administrative privileges. If necessary this can be started using the Start button -> All apps -> Windows System -> Command Prompt (right click -> More -> Run as administrator). The security question has to be answered with a click on "Yes".
In the Command Prompt execute the following commands one after another to reset the DNS cache and renew the IP configuration: netsh int ip delete arpcache , ipconfig /flushdns and ipconfig /renew .
Administrative privileges are required for this. The Device Manager can be found using the Start button -> All apps -> Windows System -> Control Panel -> Hardware and Sound -> Device Manager. In the menu activate View -> Show hidden devices so all devices are visible. Then in the Network adapters section right click each WAN Miniport adapter and uninstall it. Afterwards in the menu choose Action -> Scan for hardware changes to reinstall the adapters again. You can also start the VPN Manager and establish a connection using IPsec to bring the WAN Miniport adapters back.