Connect to PPTP over Softether

[SoftGuest]

Hello, from the client (Windows PC) I need to establish a connection to server A running SoftEther and then through this connection I need to connect to Server B using PPTP protocol. Client and server A are under my control, server B is not.
I tried basic softether VPN set up with Virtual NAT; connection has been successful, the public IPv4 address of the client got changed to that of the server A.
However, trying to establish PPTP session after that results in connection time-out.
I thought that since SoftEther is L2 it could encapsulate GRE packets (used by PPTP). Maybe there's something I can do to achieve this. Also I'd appreciate if you can suggest any other solution even if it is a bit off-topic and uses other software/protocol.

P.S. Please do not advice getting rid of the PPTP protocol, for now it is not possible for me.

[solo]

Try the following steps:

1. on server A set DisableKernelModeSecureNAT = 1 and restart it
2. from server A connect to B with PPTP and test ping to B's subnet
3. connect a SE client to server A
4. from the client ping B's subnet

A, vNAT, B and the client must be on unique subnets.