I got my Windows 8.1 updated on Lenovo T440s Laptop the other days, and found my personal VPN unsuccessfully connected. What’s weired is my Android and IOS devices connected normally. So I reinstalled my PPTP and L2TP based on IP-SEC, and it turned out that the same issue with me. What’s going on?

First of all, I double checked that all different devices had been able to connect my VPN server, so it’s not about my VPS. Secondly, I updated win8 which may be quite the cause of abnormity. Thirdly, it verified that my Ubuntu client connected successfully.

So the anwer is clear, it has nothing to do with my VPS, but about the win8. Actually, (the following solution came from the internet) Microsoft has errors with the newest WAN MINI port drivers since Jan. 2013 which I found it  in a forum, and MS shouldn’t solve it! So the solutions are as follows:
1) Uninstall the drivers related to WAN Miniport (IP), WAN Miniport (IPv6) and WAN Miniport (Network Monitor).
2) Start -> Run ->  Regedit -> HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class{4d36e972-e325-11ce-bfc1-08002be10318} -> Export
3) Edit the exported regist file and locate the above three subkey and deleted them all, double check you don’t delete the wrong part. Those subkeys has a “DriverDesc” which matches your broken miniports.
4) Go back to Device Manager, and now you are able to update the WAN MINI PORT drivers with errors. Right click the WAN MINI PORT IP (4 eg) -> Update Driver Software -> Browse My Computer -> Let me pick driver from a list -> uncheck “Show compatible hardware” and wait for the drivers listing generation. Then choose the first “Microsoft” drivers listed and pull the polling row to the top and then choose the first [BlueTooth Personal], and ignore warnings. Then the driver will become a fake blue tooth driver and you can delete it now.
5) Repeat step 4 for the remaining  WAN MINIport IPv6 and  WAN MINIport (Network Monitor)
6) Reboot, then all the right drivers will be installed automatically.