Update to 4.2.4 causes "502 Bad Gateway"

I installed a clean version yesterday on a brand new droplet (Debian 8 x64). I used the redis installation.

I updated to 4.2.4 today and since, when I update content on pages I get : 502 Bad Gateway

Everything else works fine, I can access the pages.

Here are the logs :

2015/08/04 09:27:46 [error] 645#0: *32 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: XXX.XXX.XXX.XXX, server: XXX.XXX.XXX.XXX request: “POST /wp-admin/post.php HTTP/1.1”, upstream: “fastcgi://”, host: “XXX.XXX.XXX.XXX”, referrer: “http://XXX.XXX.XXX.XXX/wp-admin/post.php?post=2367&action=edit&lang=fr