Microsoft is pushing updates for almost every Tuesday. The updates generally include minor fixes rather than big changes. But the latest Thursday updates which are named KB5009543 for Windows 10 and KB5009566 for Windows 11 caused a big change for L2TP VPN users; breaking their VPN connections.
“Can’t connect to VPN”
Because of a bug that comes with those updates, the users of L2TP VPN users faced an error, starting with “Can’t connect to VPN”. The integrated Event Log for Windows also shows the error code of 789 with the same warning: VPN failed. There have been reports of failure of connections to Ubiquiti Client-to-Site VPN, Sonicwall, Cisco Meraki, and WatchGuard Firewalls on social media. Microsoft confirmed the current bug and it will be releasing a new patch to solve the issue.
VPN error 789 solutions for Windows 10 and Windows 11
The company stated that disabling the Vendor ID (if possible) on the VPN server might serve as a temporary workaround. If the VPN service you are using does not support disabling Vendor ID, you can uninstall the related Windows Updates via command prompt.
Currently, the only solution to reconnect to those VPN servers other than disabling Vendor ID is to remove the latest cumulative updates, then restart the PC. To remove the related updates, you can use the command lines below on the elevated command prompt.
Users should keep in mind that, doing this process will uninstall the entire update, including the security-related ones. Therefore, your PC will be missing the latest security patches and might be vulnerable. Consider the risk before uninstalling the aforementioned updates.
For Windows 10:
wusa /uninstall /kb:5009543
For Windows 11:
wusa /uninstall /kb:5009566