I have also started to see these errors in my log -- readv() failed (104: Connection reset by peer). I recently upgraded from nginx 1.6.2 to 1.8, and I think they started from this point.
This is with a Linux/PHP-FPM setup. I've found a few mentions of the above error with this setup, but they're all related to back-end timeouts. None of my requests to the back-end run for more than a second or two, so I don't think this is a timeout issue for me.
Has anyone else seen this issue occur recently, or can provide any pointers?