我将我的服务器从Ubuntu 15.10更新到Ubuntu 16.04，更新的内容之一是PHP。升级后，通过ssh进行插件更新不再有效。

起初，它会提示输入FTP登录凭据，直到我安装了php-ssh2（注意它是基于git快照的）。现在，当我尝试升级插件时，它会说

An error occurred while updating WP to Twitter: Unable to locate WordPress Content directory (wp-content).

和in/var/log/apache2/error。日志：

[Sun Jun 12 11:01:10.969931 2016] [:error] [pid 5545] [client 192.168.1.11:65278] PHP Warning:  file_put_contents(): Unable to open ssh2.sftp://Resource id #81/.maintenance on remote host in /home/blog/wordpress/wp-admin/includes/class-wp-filesystem-ssh2.php on line 252, referer: http://xxxx.com/wp-admin/update-core.php?action=do-plugin-upgrade
[Sun Jun 12 11:01:10.970772 2016] [:error] [pid 5545] [client 192.168.1.11:65278] PHP Warning:  file_put_contents(ssh2.sftp://Resource id #81/.maintenance): failed to open stream: operation failed in /home/blog/wordpress/wp-admin/includes/class-wp-filesystem-ssh2.php on line 252, referer: http://xxxx.com/wp-admin/update-core.php?action=do-plugin-upgrade

我是否缺少一个包或配置项，或者什么？