just forgot one point,

the nginx error log is full of lines like this:
2013/05/05 12:34:25 [error] 5388#4772: *35 upstream timed out (10060: FormatMessage() error:(15105)) while connecting to upstream, client: 127.0.0.1, server: backend.myserver1.de, request: "GET /index.html HTTP/1.1", upstream: "http://127.0.0.1:10001/index.html", host: "backend.myserver1.de"