The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer.
January 14, 2022
Problem: Microsoft had installed an update that breaks the L2TP VPN connection.
Solution: Open an administrative command prompt.
Type: wusa /uninstall /kb:5009543 for Windows 10
Type: wusa /uninstall /kb:5009566 for Windows 11
https://community.ui.com/questions/Windows-update-breaks-L2TP-VPN-Connections-Windows10-KB5009543-Windows11-5009566/4c4d2ee3-3733-49d8-af61-4d29bfbdf75a
https://community.ui.com/questions/VPN-ISSUE-KB5009543/08358b3c-f432-440b-b891-7ef924599bab
Fix:
2022-01 Cumulative Update for Windows 10 Version 21H2 for x64-based Systems (KB5010793) January 17, 2022