enfascination the "Cannot GET /notes" message is coming from your NodeJS app. It's throwing that error because the app does not "know" that it is running on a sub-path instead of the root.
On a lot of Node.js apps (well, specifically Express apps) you can achieve this by setting up the router in the code like this:
app.use('/notes', router);
However, assuming this if for your Etherpad app I've not found a way to make that work with Etherpad.
As an alternative you can run Etherpad on a separate site using a subdomain like notes.enfascination.com instead of on a sub-path like enfascination.com/notes.