I have LMDE (3.2.0-3-amd64) with Cinnamon 1.6.4
Also installed (all with ver 0.9.4.0):

openvpn
network-manager-openvpn
network-manager-openvpn-gnome

I configured an openvpn connection with Network GUI to a vpn-server (that I don't control).
Downloaded:

xxx.se.ca.crt
xxx.ta.key
xxx.ovpn

from the server provider which I used in the configuration.
But I receive an error when trying to connect ... invalid VPN secrets.
Have restarted openvpn, network-manager and even rebooted ... still the same sad error.

Is the error on my side or on the server side ?
How can I debug ... I am starting the connection with Network GUI icon in the panel.

Are you missing a key file? You appear to have the Certificate Authority certificate and the client key, but you also need the client certificate.

Where can I get the client certificate ?
The files:
xxx.se.ca.crt
xxx.ta.key
xxx.ovpn
were the only files I got from the VPN provider.

The client cert is should be provided by the VPN provider, I would ask them. It's possible they have a different setup that doesn't require the client cert.

I will check with them ...