Reverse proxy not working

I am trying to create a reverse proxy using Apache. I'm using Apache to serve a php application and I have written an API in node which uses Express.

Inside my PHP application I use AJAX calls to node to retrieve JSON. I want to use port 80 for the calls in the PHP app and a reverse proxy using Apache to Express.

The issue I have is that I get an error 500 (proxy request).

My suspicion is that it has something to do with SSL.

Here's part of the configuration I have for Apache:

SSLEngine On
ProxyPreserveHost On
SSLProxyEngine On
ProxyPass /stats https://127.0.0.1:8081/
ProxyPassReverse /stats https://127.0.0.1:8081/
ProxyRequests on

Here's how I configured node:

app.listen(8081, function () {
  console.log('Listening on port 8081!')
})

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