API calls from the server require an appsecret_proof argument
The only way that I was able to put it working, was by settings to No the App Secret proof for API calls in the advanced settings of the facebook App.
However, this is a fix, not a solve, since I wasn't able to do the request in the option set to Yes (as is possible in the iOS facebook sdk).
you need to add a parameter "appsecret_proof" to your request containg a 'sha256' hash of accessToken and appSecret
https://developers.facebook.com/docs/graph-api/securing-requests
You need to disable Require App Secret in facebook app advance settings.