Same here. I have identified the issue but not the solution.
I use 3 Apps. One of them today started to fail. The problem is that facebook is requiring appsecret_proof.
Anyone with the same problem can check here:
https://developers.facebook.com/tools/explorer
More Info:
https://developers.facebook.com/docs/graph-api/securing-requests
I don’t have any option to disable this on API.
