Using OpenVPN Server on Linux as well as Windows machine, you may encounter this error trying to connect a client :
Unable to obtain Session ID from “vpn.domain.tld”, port(s)=443: XML-RPC: TimeoutError
Unable to obtain Session ID from “vpn.domain.tld”, port(s)=443: XML-RPC: ‘400’, ‘Bad Request’
This happen because HTTPS (SSL) port (TCP-443) is closed either on the server or firewall (server-side).
You also need the following ports open in order to get OpenVPN working :
TCP-943
TCP-1194
UDP-1194