After the upgrade Debian 7 to Debian 8 on OpenVZ platform on OVH the OpenVPN service stopped working. The problem was that the “TUN” device was missing, so the command below will temporary fix this problem.
root# mkdir -p /dev/net
root# mknod /dev/net/tun c 10 200
root# chmod 600 /dev/net/tun
Also you can add this to a script at start up.