I did a fresh install a flarum. Added FB app keys, enable the FB app, add app domain and call back.
1 - I click to "Log in with FB"
2 - Accept the login (on the FB pop-up)
3 - In the pop-up facebook redirect mydomaine.com/auth/facebook?code=XXX
==> 500 Internal Server Error. Something went wrong on our server. (in the pop-up)
And I have nothing on my server log!