SoftEther VPN User Forum

Try to change the encryption algorithm on the server and connection settings on your client so as to make it similar to browsing a normal web page. Softether VPN is using HTTPS in fact.

based on my experience, it's because the GFW detected that your connection to the IP with SE protocol.
I used to set up a SE server on Taiwan VPS and once I connected to it using SE server management, the VPS's IP became totally unreachable.
Luckily I have VPS inside China, so now my solution is let the SE server in Taiwan cascading to the server in China, and I connect to the server in China instead. the GFW only filter packets outgoing.