Registry fix for Windows 10/11 L2TP VPN

Launch Command Prompt as admin.

Paste the following (these are all single lines!):
REG ADD HKLM\SYSTEM\CurrentControlSet\Services\PolicyAgent /v AssumeUDPEncapsulationContextOnSendRule /t REG_DWORD /d 0x2 /f

Once the operation completes successfully, also paste (optional):
REG ADD HKLM\SYSTEM\CurrentControlSet\Services\RasMan\Parameters /v ProhibitIpSec /t REG_DWORD /d 0x0 /f

Restart the PC.