The status code 413 usually happens when someone tries to upload a file that is too big. You can solve this problem by changing the nginx configuration.
Open nginx configuration located at /etc/nginx/nginx.conf, search for the string client_max_body_size and then change the max body size:
Or just use the value
0 to disable max body size:
If you don't have the client_max_body_size on the configuration, you can just add a new line with the new max boy size.
Now you can reload nginx with:
service nginx reload