commerce_kickstart cart not updating

hey,

we host a few sites on commerce_kickstart (versions : 7x-1.17 & 7x-1.20).
we had a problem with all of those sites with cart not being updated. with a simple add to cart, cart doesnt get updated and one has to reload the page to see renewed cart contents.
after trying a very simple nginx config, that error was gone, so we started testing with nginx_simple_include.conf. the option that made the difference was this one :
https://github.com/omega8cc/nginx-for-drupal/blob/master/aegir/conf/ngin...
(option : fastcgi_cache_valid 200 10s; )

after commenting out only the obove line, cart works as expected.
setting the same limit to 1s instead of 10s, also works as expected.

when we hit "add to cart" button, there are 2 requests in access.log :
xxx.xxx.xxx.xxx - - [09/Dec/2013:14:49:48 +0200] "POST /bags-cases/go-green-drupal-commerce-reusable-tote-bag HTTP/1.1" 302 5 "http://commtest.tux.stinpriza.org/bags-cases/go-green-drupal-commerce-re..."
xxx.xxx.xxx.xxx - - [09/Dec/2013:14:49:49 +0200] "GET /bags-cases/go-green-drupal-commerce-reusable-tote-bag HTTP/1.1" 200 6642 "http://commtest.tux.stinpriza.org/bags-cases/go-green-drupal-commerce-re..."

that link is from a test site, you can check for yourselves, its using the original config with the problem.
i could provide debug.log as well if needed.
thanks in advance,
d.