Due to a security fix in the openvpn package, an update is available for your Debian installation. You need to upgrade as soon as possible:
Using the web administration ( https://YOUR-DOMAIN.COM/yunohost/admin/ ) then go to Tools > Upgrade, and click on the “Upgrade packages” button.
Or with this command line :
$ apt-get update && apt-get dist-upgrade && yunohost app upgrade