L2TP VPN connection Bug Surfaces after patch day of Windows 10 and 11

After the January Patch Day, the first users reported problems with VPN connections. A bug is probably preventing the establishment of L2TP VPN connections for Windows 10 and Windows 11, the cause is still unclear. This emerges from various reports from those affected who have the Bleeping Computer has collected.

Windows 10 users and administrators report sudden problems establishing L2TP VPN connections, which only occurred after installing the new cumulative updates KB5009543 for Windows 10 and KB5009566 for Windows 11. According to Reddit, the error message “” The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer “says that the L2TP connection attempt failed because a processing error was detected.

These updates were released on January 11th, the first regular patch day of the year. Microsoft has not yet named any such problems as known bugs in the support documents associated with the updates. Microsoft has other Windows 10 updates to fix security gaps and errors but apparently not affected. Problems with VPN are reportedly only in connection with Windows 11 and the newer Windows 10 versions 2004, 20H1 and 21H1.

Previous Updates interrupt L2TP connections

After installing the new patches, connections to the Windows VPN client fail every time. Entries are logged in the event log with error code 789, which only states that the connection to the VPN failed. However, only Windows’s own VPN client is affected; according to user reports, third-party solutions do not have the problem.

The use of Windows VPN has also increased more due to the pandemic, as more users are working from home and secure connections are also dependent. Microsoft has not yet confirmed the bug and has not yet responded to initial media inquiries. A temporary solution has also not yet emerged – anyone who depends on the L2TP VPN connection can currently only restore the function by uninstalling the latest patch.