nginx 500 error, permission denied for tmp folder
Phew. Solved. As the error message says, this indeed was just a "Permission" issue.
Check through "/var/lib/nginx/tmp/client_body/" and make sure the permission is correct at each directory level solve the issue.
More details can be found here : http://derekneely.com/2009/06/nginx-failed-13-permission-denied-while-reading-upstream/ and here : Permission Denied error with Django while uploading a file