This happens from time to time in Windows. I'm writing this because it just happened to one of our internal systems where L2TP had been working fine forever. Another system SSTP was broken with an error 720. Follow these steps to fix the issue.
Run an elevated command prompt. Start, type "cmd", right-click, Run as Administrator. Then enter the following commands:
netsh int ip reset
netsh int ipv6 reset
netsh winsock reset
Reboot the computer and attempt your connection again. Note you may have to re-enter any static IP information in your Ethernet/Wi-Fi adapter as well.
Open the Device Manager. Expand "Network adapters"
Uninstall all WAN Miniports, starting with Network Monitor, including IKEv2, IP, IPv6, L2TP, PPTP, SSTP and PPPoE.
Right click your computer in Device Manager, and select "Scan for hardware changes"
Try your connection again.
For L2TP, if you still have issues, try adding this registry entry below:
Run an elevated command prompt. Start, type "cmd", right-click, Run as Administrator. Then enter the following command:
REG ADD HKLM\SYSTEM\CurrentControlSet\Services\PolicyAgent /v AssumeUDPEncapsulationContextOnSendRule /t REG_DWORD /d 0x2 /f