Microsoft’s first Patch Tuesday of 2022 seems cursed with problems. Users report that KB5009543 for Windows 10 2004, 20H1 and 21H1 is causing problems establishing a VPN connection for a number of clients and servers over the L2TP VPN protocol.
Users report that they are in the “Cannot establish a VPN connection. The L2TP connection attempt failed because the security layer encountered a processing error during the initial negotiations with the remote computer“, And primarily affects the built-in Windows VPN client when connecting to Ubiquiti Client-to-Site VPN, SonicWall, Cisco Meraki, and WatchGuard firewalls.
Microsoft has confirmed the problem with the words:
“After installing KB5009543, IP security connections (IPSEC) with a vendor ID may fail. VPN connections that use Layer 2 Tunneling Protocol (L2TP) or IP Security Internet Key Exchange (IPSEC IKE) could also be affected. “
The main workaround is to uninstall KB5009543 by going to Control Panel> Programs -> View Installed …