systemd doesn't seem to handle OpenVPN's optionally-interactive service properly. If you're using a VPN like Red Hat's, you need to enter the password manually when starting the service, as it has a component which changes every time. The OpenVPN service is written to handle this, and prompt you for a username and password, but this doesn't seem to work with systemd: it just fires off the service non-interactively and it fails because the password is incorrect.

If I run openvpn --config /etc/openvpn/redhat.conf --daemon , it works okay.