@Kulga alright, just reinstalled. Everything is owned by the Nginx user with no permissions modified.
Additionally, I set my php.ini to display errors, and this is what I get:
Warning: file_put_contents(/var/www/sub.website.com/flarum/storage/framework/views/bb70210d87a252556c175ac77e7b5232): failed to open stream: Permission denied in /var/www/sub.website.com/flarum/vendor/illuminate/filesystem/Filesystem.php on line 81
Warning: file_put_contents(/var/www/sub.website.com/flarum/storage/framework/views/f8aebd3f27d63d7f2545b724c2ea9039): failed to open stream: Permission denied in /var/www/sub.website.com/flarum/vendor/illuminate/filesystem/Filesystem.php on line 81
Warning: include(/var/www/sub.website.com/flarum/storage/framework/views/f8aebd3f27d63d7f2545b724c2ea9039): failed to open stream: No such file or directory in /var/www/sub.website.com/flarum/vendor/illuminate/view/Engines/PhpEngine.php on line 42
Warning: include(): Failed opening '/var/www/sub.website.com/flarum/storage/framework/views/f8aebd3f27d63d7f2545b724c2ea9039' for inclusion (include_path='/etc/php.d') in /var/www/sub.website.com/flarum/vendor/illuminate/view/Engines/PhpEngine.php on line 42