I recently started getting the above error when trying to connect to my dappnode via openvpn. I accessed the server by IP addess, ie: http://192.168.0.25:8092/?id=XXXXX , and downloaded the .ovpn file, but still got the same error.
ssh into your dappnode server, and switch to the root user.
cd /usr/src/dappnode/DNCORE/
docker-compose -f docker-compose-vpn.yml down -v
docker-compose -f docker-compose-vpn.yml up -d
Make sure you use the -v option on the down command. This removes the old volumes, where the Certificate was stored. The -d option on the up command starts the container detached, so that it doesn’t shut down when you log out.