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.