Just recently got a r7800, would like to set up VPN. I was stuck here for two days.
The warning popped up this, "Insufficient permissions to read UCI configuration."
See manual how to configure AirVPN, and configure it in the same manner: https://airvpn.org/forums/topic/20303-airvpn-configuration-on-openwrt-preventing-traffic-leakage-outside-tunnel/