The package softethervpn 4.25-9656-1 for Opal (SFT1200 firmware 3.216) is incompatible with the openssl version OpenSSL 1.1.1i:
root@GL-SFT1200:~# service softethervpnclient start
-- Alert: RsaCheck() --
OpenSSL Library Init Failed. (too old?)
Please install the latest version of OpenSSL
It is due to SoftEther trying to create RSA test keys of 128bits with a hardened OpenSSL supporting only 1024bits minimum. SoftEther developers fixed in with this pr /SoftEtherVPN/pull/481).
According to the changelog, SoftEther VPN 4.34 Build 9745 RTM should be the most compatible version for current OpenSSL lib version (1.1.1l) shipped with v18.06.5.