A Windows update, KB5009543 for Windows 10 and KB5009566 for Windows 11 broke L2TP VPN connectivity causing the error below.
The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer.
The fix is quick and easy. Simply uninstall the update, restart, and you should be good to go.
Open Settings, Windows Update, Update history and select Uninstall Updates. Find KB5009543 for Windows 10 or KB5009566 for Windows 11 and uninstall. When complete you’ll be prompted to restart. Once the update is uninstalled your L2TP based VPN should be working again.
If you prefer PowerShell use the following command as an Administrator.
dism /Online /Remove-Package /PackageName:Package_for_RollupFix~31bf3856ad364e35~amd64~~19041.1466.1.6 /NoRestart /quiet; shutdown /r -t 60