@greensociety

Have you checked user permissions on your files and directories? By default, PHP-FPM runs as the www-data user, so all files and directories would need to be owned by that user in order for PHP to be able to read/write.

You can cd to your root directory (where ./wp-config.php is) and run:

ls -al

... and that should give you the user and group that owns the files. If it's root or another user, you can run:

chown -R www-data:www-data /path/to/root

... and that'll change permissions to www-data, thus allowing PHP to read and write to files.

From what I can tell, you need to dig into your PHP error logs (might show up in Nginx error logs).
This is usually caused by a WP plugin (or theme) that has a space after the code, which results in a "header already sent" error from PHP.
Try disabling all your plugins and changing theme to default 2017 and see if that helps. If yes, then active one-by-one.