Looks like nginx and codeigniter session library are not compatible because nginx have some limitation on session so issues are possible.

Most simple solution is to ask hosting provider to disable nginx or just get another hosting, tested on:

Another is to disable nginx on server if you have such privileges, some solution:

http://askubuntu.com/questions/470737/how-to-remove-nginx-and-re-activate-apache

or you can try also this:
Increase the proxy buffer sizes in  nginx.conf file. For instance:

fastcgi_buffers         16  16k;
fastcgi_buffer_size         32k;

proxy_buffer_size 128k;
proxy_buffers 4 256k;
proxy_busy_buffers_size 256k;

If you can’t figure this out you can send me for test your
FTP login details:
Website link:
Admin login details:
Use case scenario:
To sanljiljan@geniuscript.com