Change your fastcgi buffers

fastcgi_buffers 16 24k; 
fastcgi_buffer_size 128k;

Change your proxy buffers

proxy_buffers   16 24k;
proxy_buffer_size   128k;
proxy_busy_buffers_size   128k;

In both cases try to find the appropriate buffer size, the sizes posted here are for illustration purposes but should work in 80% of cases.

Also note that in case of long polling services you should turn off the proxy buffers using:

proxy_buffering off;