L2TP VPN connections broken after update (KB5009543 & KB5009566)

Windows users find their L2TP VPN connections broken after install 2022-01 Cumulative Update (KB5009543 & KB5009566)

According to Microsoft’s documentation, the VPN connection issues affected “IPSEC connections which contain a Vendor ID,” as well as L2TP and IPSEC IKE VPN connections in Windows 10, Windows 11, and Windows Server versions 2022, 20H2, 2019, and 2016. Windows’ built-in VPN client seems to be the most commonly affected, but third-party VPN clients using these kinds of connections could also run into the error.

On the computer screen, an error says “Can’t connect to VPN. The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer” and you can see the Event ID 20227 with “error 789” on the application event viewer.

How do I fix the VPN error 789 on Windows 10 & 11?

Solution 1: 

Step 1: Launch Powershell with admin rights via the search bar.

Step 2: In the Powershell window, type wusa /uninstall /kb:5009543 or Windows 11: wusa /uninstall /kb:5009566 and press Enter.

Step 3: Restart Windows.

Solution 2: 

Thankfully now, the issue can be resolved with an out-of-band update labeled KB5010793 for Windows 10 and KB5010795 for Windows 11

Windows 10: https://www.catalog.update.microsoft.com/Search.aspx?q=KB5010793

Windows 11: https://www.catalog.update.microsoft.com/Search.aspx?q=KB5010795