Para usar os certificados letsencrypt no openvpnas, você precisa ter os seguintes links:
/usr/local/openvpn_as/etc/web-ssl/ca.crt -> /etc/letsencrypt/live/my.domain/fullchain.pem
/usr/local/openvpn_as/etc/web-ssl/server.crt -> /etc/letsencrypt/live/my.domain/cert.pem
/usr/local/openvpn_as/etc/web-ssl/server.key -> /etc/letsencrypt/live/my.domain/privkey.pem