Comment # 5 on bug 1031748 from Robert Simai

I can confirm the behavior from the initial comment on bare metal Leap 42.2. I
get a prompt (or gui window) to check the root credentials, then it's stuck
until timeout without asking for username/password.

I however (have an alias since ages) use

 $ sudo systemctl start openvpn@MYVPN.service

which works just fine while it seems systemd should be able to handle it also
for non-root users.