Earlier this week, Microsoft released another cumulative update for supported versions of Windows, which eliminates a number of dangerous vulnerabilities and also brings fixes aimed at improving the stability of the software platform. Now it has become known that the update can cause problems with the VPN connection in Windows 10 and Windows 11.
This is indicated by user contributions that have appeared in the Microsoft support forum and in various communities such as Reddit. In some cases, users are unable to establish a VPN connection with the built-in Windows client after downloading the mentioned update. A message appears when trying to connect: “Unable to connect VPN. The L2TP connection attempt failed due to an error that occurred in the security layer when negotiating with the remote computer.”. At the same time, the Windows Event Viewer throws an error with code 789 and reports that the connection could not be established.
According to the source, the problem is most common when using the built-in Windows VPN client, but it can also appear when interacting with third-party solutions like Ubiquiti, SonicWall, Cisco Meraki, etc. Currently the easiest way to solve this problem is to remove the last cumulative update. To do this, open the update history of Windows Update and remove the required package from the list (KB5009566 for Windows 11 and KB5009543 for Windows 10).
Apparently, Microsoft will be releasing an unscheduled patch soon to fix the mentioned issue. When this could be the case is currently unknown.