Is there any proxy (other machine) between VPS and Facebook?
I made Facebook Login button for a site, on localhost it works but on DO VPS it doesn't. I am thinking that requests from VPS -> Facebook are changed, goes through another machine before reaching Facebook. Here are all details https://github.com/facebook/facebook-php-sdk-v4/issues/476
I don't know what to do and what/how to debug this problem.