Reason:

After enabling strict mode and force HTTPS redirect URI, login process is not working for weird reasons. Here are some steps required:

Error: cross domain issue

1. composer update
2. Change default version to v2.10
3. Add this script after creating a helper: (Very important!!)
   
   if (isset($_GET['state'])) { $helper->getPersistentDataHandler()->set('state', $_GET['state']); }

   1 2 3

   if (isset($_GET['state'])) {     $helper->getPersistentDataHandler()->set('state', $_GET['state']); }

Reference:

• https://stackoverflow.com/questions/47898499/error-on-login-using-facebook-php-sdk-with-strict-mode-enabled
• https://stackoverflow.com/questions/48528231/fb-login-strict-mode-is-not-working
• https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow/#logindialog