UPDATED: Out-of-band update that resolves this issue was released by Microsoft and can be downloaded. We have confirmed that the issue is resolved. Respective KBs for Windows 11: KB5010795 or for Windows 10: KB5010793. These updates are readily available to download from the Windows Update service. The following text is present purely for backlog purposes and can be treated as outdated.
It has been reported to us that customers who have already deployed cumulative updates from 11th January 2022 KB5009543 or KB5009566 for Windows 10 and Windows 11 respectively suffer from an inability to connect to their L2TP or IKEv2 connections any more. After thorough investigation, there is currently no other solution available except removal of the offending updates and waiting for Microsoft to resolve the issue via upcoming updates.
To remove the update, open the command prompt and input the following command.
Right-click Start and choose Command Prompt or Command Prompt (Admin) from the Quick Link menu. You can also use keyboard shortcuts for this route: Windows key + X, followed by C (non-admin) or A (admin). Type cmd in the search box, then press Enter to open the highlighted Command Prompt shortcut.
wusa /uninstall /kb:5009543
wusa /uninstall /kb:5009566