message: middlewareError - nodejs

I uploaded a nodejs application to an ec2 AWS ubuntu instance. After some time online it doesn't respond anymore.

Now I installed winston to log errors and got the following:

{
  "date": "Sat May 20 2017 19:41:23 GMT+0000 (UTC)",
  "process": {
    "pid": 6740,
    "uid": 1000,
    "gid": 1000,
    "cwd": "/home/ubuntu/www",
    "execPath": "/usr/bin/nodejs",
    "version": "v4.2.6",
    "argv": [
      "/usr/bin/nodejs",
      "/home/ubuntu/www/server/app.js"
    ],
    "memoryUsage": {
      "rss": 95809536,
      "heapTotal": 68333664,
      "heapUsed": 65639712
    }
  },
  "os": {
    "loadavg": [
      0.046875,
      0.04931640625,
      0.0263671875
    ],
    "uptime": 33009
  },
  "trace": [],
  "req": {
    "url": "/users/me",
    "headers": {
      "host": "api.example.com",
      "connection": "keep-alive",
      "accept": "application/json, text/plain, */*",
      "origin": "http://example.com",
      "user-agent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36",
      "referer": "http://example.com/",
      "accept-encoding": "gzip, deflate, sdch",
      "accept-language": "en-US,en;q=0.8"
    },
    "method": "GET",
    "httpVersion": "1.1",
    "originalUrl": "/users/me",
    "query": {}
  },
  "level": "error",
  "message": "middlewareError"
}

The error message middlewareError is not really clear to me. It seems like there is a memory problem? If thats the case, can anyone point me in the right direction how to dig into this?

via Stefan