If you cannot start OpenVPN on your VPS with Ubuntu 18.04, Debian 9 or CentOS 7 with the following message: "openvpn_execve: unable to fork: Resource temporarily unavailable (errno=11)"
Please do the following:
Run "sudo systemctl edit openvpn-server@" on SSH. This will open up nano editor with an empty page.
Then add the following:
Finally, run "systemctl daemon-reload"