@tisto I didn't touch it. I'm sure that's the reason for the issue. For development that feels a bit much though.
The following got everything working for me :
PORT=3000 CLIENT_PUBLIC_PATH=http://$C9_HOSTNAME:3001/ RAZZLE_API_PATH=http://$C9_HOSTNAME:3000/api yarn start
I've updated the first post in this thread to reflect the proper solution. Now I can do Cloud9 based Volto development!!!!