Private key incorrect

Solution found! If you have a plain vanilla Debian 7 server like me, follow this nice guide:

https://www.serverstack.com/blog/2013/02/11/automatic-wordpress-updates-using-ftpftps-or-ssh/

Instead of saying
define(‘FTP_HOST’,’127.0.0.1:22′);
say
define(‘FTP_HOST’,’www.example.com:22′);

Do not use PECL, install the Debian version instead:

apt-get install libssh2-php

Result: WP will never again ask for a ssh key. ??