We are runnging nginx reverse proxy on windows, the upstream consists of two lunixes based Apache/PHP Backend Servers. There is only one PHP application to be proxied. After starting nginx everything works fine, but then the backends become partly unresponsive and nginx is logging

2016/05/24 18:42:27 [warn] 5116#4660: *8863 upstream server temporarily disabled while reading response header from upstream, client: 77.23.138.88, server: wahl2.hannover-stadt.de, request: "GET / HTTP/1.1", upstream: "http://192.168.57.14:80/wrs/", host: "wahl2.hannover-stadt.de"
2016/05/24 18:42:27 [error] 5116#4660: *8863 upstream timed out (10060: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond) while reading response header from upstream, client: 77.23.138.88, server: wahl2.hannover-stadt.de, request: "GET / HTTP/1.1", upstream: "http://192.168.57.14:80/wrs/", host: "wahl2.hannover-stadt.de"
2016/05/24 18:42:36 [warn] 5116#4660: *8862 upstream server temporarily disabled while reading response header from upstream, client: 77.23.138.88, server: wahl2.hannover-stadt.de, request: "GET / HTTP/1.1", upstream: "http://192.168.57.14:80/wrs/", host: "wahl2.hannover-stadt.de"
2016/05/24 18:42:36 [error] 5116#4660: *8862 upstream timed out (10060: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond) while reading response header from upstream, client: 77.23.138.88, server: wahl2.hannover-stadt.de, request: "GET / HTTP/1.1", upstream: "http://192.168.57.14:80/wrs/", host: "wahl2.hannover-stadt.de"
2016/05/24 18:42:37 [warn] 3560#5948: *8870 upstream server temporarily disabled while reading response header from upstream, client: 77.23.138.88, server: wahl2.hannover-stadt.de, request: "GET /modules/setflashsession.php?set=0 HTTP/1.1", upstream: "http://192.168.57.14:80/wrs/modules/setflashsession.php?set=0", host: "wahl2.hannover-stadt.de", referrer: "http://wahl2.hannover-stadt.de/"
2016/05/24 18:42:37 [error] 3560#5948: *8870 upstream timed out (10060: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond) while reading response header from upstream, client: 77.23.138.88, server: wahl2.hannover-stadt.de, request: "GET /modules/setflashsession.php?set=0 HTTP/1.1", upstream: "http://192.168.57.14:80/wrs/modules/setflashsession.php?set=0", host: "wahl2.hannover-stadt.de", referrer: "http://wahl2.hannover-stadt.de/"
2016/05/24 18:42:47 [warn] 3560#5948: *8870 upstream server temporarily disabled while reading response header from upstream, client: 77.23.138.88, server: wahl2.hannover-stadt.de, request: "GET /modules/setflashsession.php?set=0 HTTP/1.1", upstream: "http://192.168.57.88:80/wrs/modules/setflashsession.php?set=0", host: "wahl2.hannover-stadt.de", referrer: "http://wahl2.hannover-stadt.de/"
2016/05/24 18:42:47 [error] 3560#5948: *8870 upstream timed out (10060: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond) while reading response header from upstream, client: 77.23.138.88, server: wahl2.hannover-stadt.de, request: "GET /modules/setflashsession.php?set=0 HTTP/1.1", upstream: "http://192.168.57.88:80/wrs/modules/setflashsession.php?set=0", host: "wahl2.hannover-stadt.de", referrer: "http://wahl2.hannover-stadt.de/"

Googeling for " upstream server temporarily disabled while reading response header from upstream" brings up a few russian links but no clue whats the reason for this. There is no load on the system, both backend servers and nginx are idle.