Microsoft January 11, 2022 update breaks L2TP VPN [solution]

Microsoft update KB5009543 breaks L2TP VPN functionality causing error message “The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer“. Investigating the log shows error 789 for rasclient.

A temporary solution is to uninstall KB50009543. Run cmd as administrator and enter:

wusa /uninstall /kb:5009543

After reboot VPN will work again.

See also this article.