I have spent significant amount of time investigating this. It is a problem with Windows using VPN with NAT-T. If you use a MacOS computer, you will be able to connect immediately. Setting the registry key was a hit or miss for me: https://docs.microsoft.com/en-us/troubleshoot/windows-server/networking/configure-l2tp-ipsec-server-behind-nat-t-device The real solution for me was actually updating Windows to the very latest patch. Along the way Windows themselves must have noticed an issues and one of the patches fixed it. Make sure to delete the VPN connection and recreate it.
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I did this and it works for me: Install network-manager-l2tp: sudo add-apt-repository ppa:nm-l2tp/network-manager-l2tp and sudo apt-get update sudo apt-get install network-manager-l2tp If using gnome, install the gnome plugin (if using another desktop environment, see if there's a plugin for its network manager): sudo apt-get install network-manager-l2tp-gnome Reboot Navigate to Settings > Network > Click the +button > Select "Layer 2 Tunneling Protocol (L2TP)" Name the new VPN connection something Put the host name or address in the Gateway field. Put username in the Username field. Click the icon in the Password field and select your preference for how to supply the password. Click IPSec Settings... Click the box for "Enable IPsec tunnel to L2TP host" Enter the shared secret into the Pre-shared key field. Leave the Gateway ID field empty. Expand the Advanced options area Enter "3des-sha1-modp1024" into the Phase 1 Algorithms box. Enter "3des-sha1" into the Phase 2 Algorithms box. Leave the box checked for "Enforce UDP encapsulation". Click OK. Click Save. Open a terminal and enter the following commands to permanently disable the xl2tpdservice: sudo service xl2tpd stop Also enter the following: sudo systemctl disable xl2tpd Open Network Settings and try to turn the VPN on.
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