The http headers returned to the client are incorrect. It should be "Connection 'keep-alive'" instead of "Conncetion "close"". Somehow all the nginx settings for response headers seem to be ignored. The fact that the application server's name is returned in the response http header instead of "Server nginx" made me think that the response is "directly" returned without taking nginx configuration into account.
The requests get logged to access.log. I enabled further log statements using "$sent_http_header" variables. Still I can't figure out why the GET requests are treated differently.