ProtonVPN over OpenVPN only routing ipv4, leaking ipv6

You are running an old and outdated build upgrading is highly recommended.
Some of the warnings are probably because you are using an older OpenVPN client

The OpenVPN log is incomplete I suspect that openpvn was already running a long time, to get more information we should see the openvpn log (logread | grep openvpn) shortly after the OpenVPN is up.
The AEAD decrypt error is actually a warning if it happens occasionally you can ignore it otherwise lower MTU to 1400.

But there is a route for IPv4 via the tunnel so the VPN seems up, but Proton does not seem to support IPv6 so you have an IPv6 leak.

To deal with this you can disable IPv6 on the router or on the clients, but if that is not feasible you can try to set an IPv6 route via the tunnel, of course this will not work so effectively it will block IPv6 not sure how your router/LAN clients will react on this.

Also not sure if your old OpenVPN client support IPv6 but you can try to add to the openvpn config: redirect-gateway ipv6 this will add the default gateway via the tunnel to your IPv6 routing table.

You can show again: ip -6 route show after you have set this in the OpenVPN config and the VPN is up