OpenVZ via its kernel architecture provides some weird operation with device nodes. I found that systemctl was unable to start OpenVPN on a CentOS 8 host which is virtualized in OpenVZ. The fix is quite simple but not so obvious

sed -i 's/LimitNPROC/#LimitNPROC/g' /usr/lib/systemd/system/openvpn-client\@.service
systemctl daemon-reload