In some rare occasion, you may be unable to access the OpenVPN WebUI through the standard port tcp-943 after a brand new installation. In my particular case, it happened once with CentOS 7.
This is caused by the init script that does not ran properly. To resolve this matter, just invoke the init script manually with the following command :
/usr/local/openvpn_as/bin/ovpn-init --force
Then you will be prompted for a bunch of questions and then the OpenVPN daemon will restart with the new init values.