OpenVPN error- write to TUN/TAP : I/O error (code=5)

I am installing ProtonVPN on OpenVPN on OpenWRT on my Netgear D7800.. lol

I have used this guide
I have configured things as it says, and at the moment I am using this opvn config file to run the VPN. I removed some info as I'm not sure if it could be compromising or not. Could someone offer some advice please?

dev tun
proto udp

remote **"PORT"**
remote **"PORT"**
remote **"PORT"**
remote **"PORT"**
remote **"PORT"**

resolv-retry infinite

# The following setting is only needed for old OpenVPN clients compatibility. New clients
# automatically negotiate the optimal cipher.
cipher **"CYPHER CODE"**

auth SHA512
verb 3

setenv CLIENT_CERT 0
tun-mtu 1500
tun-mtu-extra 32
mssfix 1450

reneg-sec 0

remote-cert-tls server
auth-user-pass /etc/openvpn/**"FILE NAME"**.auth

script-security 2
up /etc/openvpn/
down /etc/openvpn/

I am getting the the following error, despite the VPN working and allowing me to access the interwebs

Have you studied the wiki page?

Protonvpn's dutch 'free server' working with instance I set up last year on 19.07 test router when I briefly checked moments ago - see link to alternative guide at bottom of above wiki page.