When in the system tray I click on the network manager, choose an openvpn connection, and tell it to connect it shows me: "Missing VPN plugin". network-manager-openvpn is installed and it's been working since years but I didn't use it in the last two months. So I guess an update of either plasma-nm or network-manager itself led to that error. After I've installed network-manager-openvpn-gnome I could start the vpn connection using nmcli but still not usting plasma-nm
I've forgot to mention. I'm using a password protected key file. So network-manager needs to ask me for the password.