Diego96Gnome System Log will give you realtime information as to why and how your VPN connection is failing.
Personnaly i have never had any issues with OpenVPN Network Manager on all editions of Trisquel 7, it simply works out of the box and ask's for your root password the first time you "import a saved OpenVPN configuration" wich is : ca.crt, ta.key, user.cert, user.key and the config file(OpenVPN Client Configuration)
I found this https://nordvpn.com/tutorials/linux/openvpn/
