New Windows KB5009543, KB5009566 updates break L2TP VPN connections

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Updates break L2TP connections. After installing windows KB5009543, KB5009566 updates, Windows users find their L2TP VPN connections broken when attempting to connect using the Windows VPN client.

When attempting to connect to a VPN device, they are shown an error stating, “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,” The Event Log will also log entries with error code 789, stating that the connection to the VPN failed.

With many users still working remotely, admins have been forced to remove the KB5009566 and KB5009543 updates, which immediately fixes the L2TP VPN connections on reboot.

Microsoft has released OOB updates to fix the Windows L2TP VPN connection issues.
All OOB updates released today are available for download on the Microsoft Update Catalog, and some of them can also be installed directly through Windows Update as optional updates.

You will have to manually check for updates if you want to install the emergency fixes through Windows Update because they are optional updates and will not install automatically.

The following updates can only be downloaded and installed via the Microsoft Update Catalog:
Windows 8.1, Windows Server 2012 R2: KB5010794
Windows Server 2012: KB5010797
Updates for these Windows versions are also available through Windows Update as an optional update:

Windows 11, version 21H1 (original release): KB5010795
Windows Server 2022: KB5010796
Windows 10, version 21H2: KB5010793
Windows 10, version 21H1: KB5010793
Windows 10, version 20H2, Windows Server, version 20H2: KB5010793
Windows 10, version 20H1, Windows Server, version 20H1: KB5010793
Windows 10, version 1909, Windows Server, version 1909: KB5010792
Windows Server 2019: KB5010791 (Released on 1/18/22)
Windows 10, version 1607, Windows Server 2016: KB5010790
Windows 10, version 1507: KB5010789
Windows 7 SP1: KB5010798
Windows Server 2008 SP2: KB5010799

the latest Windows Server updates were causing a series of severe issues for administrators.

According to admin reports, Windows domain controllers were being plagued by spontaneous reboots, Hyper-V was no longer starting on Windows servers, and Windows Resilient File System (ReFS) volumes were no longer accessible after deploying the January 2022 updates.

Windows 10 users and administrators also reported problems with L2TP VPN connections after installing the recent Windows 10 and Windows 11 cumulative updates and seeing “Can’t connect to VPN.” errors.

Those who cannot immediately install today’s out-of-band updates can remove the KB5009624, KB5009557, KB5009555, KB5009566, and KB5009543 updates causing these issues from an Elevated Command Prompt with the following commands:
Windows Server 2012 R2: wusa /uninstall /kb:5009624
Windows Server 2019: wusa /uninstall /kb:5009557
Windows Server 2022: wusa /uninstall /kb:5009555
Windows 10: wusa /uninstall /kb:5009543
Windows 11: wusa /uninstall /kb:5009566