Have you checked that PHP-FPM is running properly etc? 502 Gateway issue can sometimes be related to FPM before it’s a script issue. If FPM etc is all working (you can test this by putting a blank index.php in your _bnew/quizzes/public/ folder with something like <?php echo 'testing'; ?> in it. If that file runs fine, then you’ll need to have a look at your PHP error logs for the issue with running Slim - it should indicate in there if something has gone wrong when instantiating the app