Can ping the VPN server, but unable to connect

[amintnt]

Hi.
I'm running SoftEther VPN Server on two different VDS servers in different locations, Iran and US; both on Windows Server 2008 R2.
Configurations on both servers are totally alike, but US server does not respond. It actually used to respond a week ago on 443 TCP port(no NAT-T), but now it doesn't. To make sure the ports are accessible from outside, I used PSping tool and there were definitely responses coming back. To double check, I used Network Traffic Speed Test Tool, each time stopping the port that was being listened to by VPN server and listening to it using the test tool, and successfully was able to connect to the ports from the client. So I'm assuming the ports are OK for a normal communication. Meanwhile the other server located in Iran is completely working fine on every port.
What could be the problem then? The server is listening on the ports and the ports are exposed to the outside world, but client gets error code 2: Protocol error occurred. Error was returned from the destination server.
Any help is appreciated.

[amintnt]

As far as I've known, the problem is due to censorship in Iran which uses Deep Packet Inspection and drops every suspicious connection, SoftEther being such. I was disappointed knowing that SoftEther had no work-around for that.