upstream prematurely closes cnx => 502 Bad Gateway to client

[Nginx User]

Nginx’ers,

I trying to figure out why I'm randomly are seeing requests having issues with a nginx 1.7.4 when proxying to an upstream pool like:

2016/03/03 10:24:21 [error] 15905#0: *3252814 upstream prematurely closed connection while reading response header from upstream, client: 10.45.69.25, server: , request: "POST /<redacted url> HTTP/1.1", upstream: "http://10.45.69.28:8081/<redacted url>", host: "mxosCl:8081"
2016/03/03 10:30:02 [error] 15905#0: *3237632 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: 10.45.69.99, server: , request: "POST /<redacted url> HTTP/1.1", upstream: "http://10.45.69.25:8081/<redacted url>", host: "mxosCl:8081"
2016/03/03 10:30:02 [error] 15905#0: *3237670 upstream prematurely closed connection while reading response header from upstream, client: 10.45.69.99, server: , request: "POST /<redacted url> HTTP/1.1", upstream: "http://10.45.69.28:8081/<redacted url>", host: "mxosCl:8081"
2016/03/03 10:30:03 [error] 15905#0: *3237670 upstream prematurely closed connection while reading response header from upstream, client: 10.45.69.99, server: , request: "DELETE /<redacted url> HTTP/1.1", upstream: "http://10.45.69.26:8081/<redacted url>", host: "mxosCl:8081"
 
upstream servers doesn't log any issues, it seems. clients get a 502 Bad Gateway for such requests.

TIA

/Steffen