Fresh installation of Windows 10, IPsec/L2TP VPN server running on QNAP or Mikrotik and problem with VPN IPsec/L2TP connection... But the problem is not QNAP or Mikrotik but Windows 10 itself.
IOS or MacOS connect without any problems - systems proven in person.
Of course, there is a quick fix for this, here we have to go to the registry editor - regdit - and add the appropriate entry:
This is to be DWORD32 called AssumeUDPEncapsulationContextOnSendRule and value 2.
After adding the entry we have to reboot Windows.