Check the encryption settings

set VPN optional data encryption on Windows 10

  1. Right-click the Start button and go to Network Connections.
  2. Click Change adapter options.
  3. Right-click your VPN connection and select Properties.
  4. Switch to the Security tab.
  5. Set Data encryption to Optional encryption (connect even if no encryption).
  6. Click OK.
If the VPN server has encryption issues, this could be the reason why you cannot connect to L2TP VPN on Windows 10. By making the encryption optional, you might be able to temporarily solve this problem.
However, encryption is a critical component of a VPN connection, so you should contact the VPN server administrator to fix this issue as soon as possible.

Check the authentication settings

set VPN security protocols on Windows 10

  • Go to Network Connections > Change adapter options.
  • Right-click your VPN and go to Properties.
  • Switch to the Security tab.
  • At Authentication, select Allow these protocols.
  • Enable Challenge Handshake Authentication Protocol (CHAP).
  • Enable Microsoft CHAP Version 2 (MS-CHAP v2).
  • Click OK.
L2TP connection issues are typically caused by invalid VPN authentication settings. Try the configuration above to see if it works. Otherwise, you should get in touch with the VPN server administrator and request assistance.