creates failing <email address hidden> unit

I tried to reproduce this in a fresh VM with:

sudo cp /usr/share/doc/openvpn/examples/sample-config-files/static-home.conf /etc/openvpn/client1.conf
sudo cp /usr/share/doc/openvpn/examples/sample-config-files/static-office.conf /etc/openvpn/client2.conf
sudo sed -i '/AUTOSTART.*"all"/ s/^#//' /etc/default/openvpn
sudo systemctl daemon-reload
sudo service openvpn restart

and it works as expected: "systemctl status openvpn@*" shows me that I got <email address hidden> and <email address hidden>. Of course they fail to start in my scenario as the example files use IP addresses 1.2.3.4 and similar, but the generator and startup works fine.

In your case <email address hidden> looks strange -- do you actually have a /etc/openvpn/client.conf file? This error message

May 1 11:45:39 ovpn-client[26561]: Options error: In [CMD-LINE]:1: Error opening configuration file: /etc/openvpn/client.conf

sounds like it either exists and is invalid, or it does not exist. Can you please give me the output of

  ls -lR /etc/openvpn/*.conf

?

For some reason commands:

sudo /etc/init.d/openvpn start
sudo /etc/init.d/openvpn restart

are not working,

but

sudo systemctl daemon-reload
sudo service openvpn restart

are

sudo apt-get purge openvpn
sudo apt-get install openvpn

fixed the problem

this report can be closed.

Well, I would have liked to know why it was failing, but I suppose the purging destroyed all the evidence now. Closing, thanks!

Well,

this part of code was never reached when invoking

sudo /etc/init.d/openvpn start

case "$1" in
start)
  log_action_begin_msg "Starting $DESC"

  # first create /run directory so it's present even
  # when no VPN are autostarted by this script, but later
  # by systemd openvpn@.service
  mkdir -p /run/openvpn

  # autostart VPNs
  if test -z "$2" ; then
    # check if automatic startup is disabled by AUTOSTART=none
    if test "x$AUTOSTART" = "xnone" -o -z "$AUTOSTART" ; then
      log_warning_msg " Autostart disabled, no VPN will be started."
      exit 0
    fi
    if test -z "$AUTOSTART" -o "x$AUTOSTART" = "xall" ; then
      # all VPNs shall be started automatically
      for CONFIG in `cd $CONFIG_DIR; ls *.conf 2> /dev/null`; do
        NAME=${CONFIG%%.conf}
        log_daemon_msg " Autostarting VPN '$NAME'"
        start_vpn
      done
    else

> this part of code was never reached when invoking
> sudo /etc/init.d/openvpn start

Yes, it's not supposed to. openvpn ships systemd units, so the init.d script isn't getting run (and if you try to call it manually it diverts to essentially calling "systemctl start openvpn").

I found a solution for this and wanted to post in case someone else came across it. OpenVPN stopped using /etc/default/openvpn for defaults after switching to systemd, even though the file /etc/default/openvpn may still be there (As it is on Debian 9 still). To configure defaults for openvpn on systemd now, issue the command:

sudo systemctl edit openvpn

If you add the AUTOSTART="client1 client2" there, and save, then it should work.